Image processing apparatus and image processing method

ABSTRACT

When a sum of a first gradation data for a first color and a second gradation data for a second color is greater than the maximum value of thresholds of a first threshold matrix, a generation unit generates a first overlapping gradation data and second overlapping gradation data by dividing a value obtained by subtracting the maximum value from the sum. Further, the generation unit generates a first quantization data based on a result of comparing the first overlapping gradation data with the second threshold or a result of comparing a difference between the first gradation data and the first overlapping gradation data with the first threshold, and generates a second quantization data based on a result of comparing the second overlapping gradation data with the first threshold or a result of comparing a difference between the second gradation data and the second overlapping gradation data with the second threshold.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an image processing apparatus and animage processing method for forming an image on a printing medium byquantization processing.

Description of the Related Art

In a case of using a half-toning method to print an image, it isnecessary to quantize multi-valued image data. As a quantization methodused in this case, an error-diffusion method and a dither method havebeen known. In particular, the dither method that compares a thresholdstored in advance with a gradation value of multi-valued data todetermine a quantization level has a smaller processing load than theerror-diffusion method, and thus is used in many image processingapparatuses. Particularly, the dispersibility of dots in a low-gradationarea may be issue in the dither method, and it has been known that gooddot dispersibility can be obtained by using a threshold matrix havingblue noise characteristics.

FIGS. 28A to 28C are diagrams for describing the dither processing usingthe threshold matrix having blue noise characteristics. FIG. 28A showsan image data example inputted to a 16×16 pixel area. This shows a statein which gradation values “36” are inputted to all the pixels. FIG. 28Bshows a threshold matrix that is prepared for the abovementioned 16×16pixel area. Any one of thresholds 0 to 255 is associated with eachpixel. In the dither method, if the gradation value indicated bymulti-valued image data is greater than the threshold, the pixel isspecified as “1” representing that a dot is printed. On the other hand,if the gradation value indicated by multi-valued image data is equal toor smaller than the threshold, the pixel is specified as “0”representing that a dot is not printed.

FIG. 28C shows a result of quantization by the abovementioned dithermethod. The pixels of “1” representing printing are in black, and thepixels of “0” representing not-printing are in white. Distribution ofthe pixels of “1” representing printing as shown in FIG. 28C depends onarrangement of the thresholds in the threshold matrix. Even in the caseof inputting the multi-valued data of equal values to a predeterminedarea as shown in FIG. 28A, use of the threshold matrix shown in FIG. 28Bhaving blue noise characteristics allows the pixels of “1” representingprinting to be arranged with high dispersibility as shown in FIG. 28C.

FIGS. 29A and 29B are diagrams that respectively show blue noisecharacteristics and visual transfer function of human (VTF) in adistance of conspicuous vision of 300 mm. In both the diagrams, thehorizontal axis represents a frequency (cycles/mm) that is lower in theleft side and higher in the right side of the graph, while the verticalaxis represents an intensity (power) corresponding to the frequency.

According to FIG. 29A, blue noise characteristics have features ofsuppressed low frequency components, rapid rise, and flat high frequencycomponents. A frequency fg at a peak of the rapid rise is called aprinciple frequency. Meanwhile, for the visual transfer function ofhuman (VTF) shown in FIG. 29B, an approximate expression of Dooley as(Expression 1) is used for example. In (Expression 1), l represents anobservation distance, and f represents a frequency:

VTF=5.05×exp(−0.138×πlf/180)×(1−exp(0.1×πlf/180))  (Expression 1).

As can be seen in FIG. 29B, the visual properties of human have highsensitivity in a low frequency area and have low sensitivity in a highfrequency area. That is, low frequency components are visuallyperceivable but high frequency components are visually unperceivable.Blue noise characteristics are made in the light of such visual transferfunction, and almost no power is applied to the low frequency area withhigh sensitivity (visually perceivable) while applying power to the highfrequency area with low sensitivity (visually unperceivable).Consequently, in a case where a human sees an image on which thequantization processing is performed with the threshold matrix havingblue noise characteristics, uneven dispersion and periodicity of thedots are hardly perceived and the image is recognized as a comfortableimage.

On the other hand, Japanese Patent Laid-Open No. 2017-38127 discloses adither method for solving a situation in which the dispersibility isdeteriorated and the granularity is conspicuous in printing of an imagewith multiple color materials (i.e., mixed-color) although gooddispersibility can be obtained with each color material (i.e.,single-color). Specifically, there is disclosed a method that prepares acommon dither matrix having good dispersibility as shown in FIG. 28B andperforms the quantization processing while shifting thresholds color bycolor for multiple colors. With this quantization processing, dots ofdifferent colors in a low-gradation area are printed mutuallyexclusively with high dispersibility, and thus it is possible to obtaina mixed-color image which is smooth with no conspicuous granularity.

It should be noted that the above-described processing has a tendencythat the dispersibility of second and following colors, which requirethe shifting of thresholds or input values, becomes inevitably lowerthan the dispersibility of a first color, which requires no shifting ofthresholds or input values. For this reason, it is preferred that thefirst color and the second color in the order of colors to be processedbe set in order from a color material with lowest lightness and highestdot power. However, in a case where there are color materials havingalmost the same dot powers such as cyan dots and magenta dots,granularity of magenta becomes conspicuous if cyan is set as the firstcolor, and granularity of cyan becomes conspicuous if magenta is set asthe first color.

In view of such circumstances, Japanese Patent Laid-Open No. 2017-38127discloses a method of performing processing by using different thresholdmatrixes for the two colors having high dot powers. Specifically, thequantization processing is performed on each of color groups, whichinclude a color group that uses the same threshold matrix as cyan andthresholds are shifted with cyan set as the first color and a colorgroup that uses the same threshold matrix as magenta and thresholds areshifted with magenta set as the first color. In this way, it is possibleto output an image in which multiple colors of dots are mixed withoutconspicuous granularity while maintaining high dispersibility of thesingle-color cyan dots and high dispersibility of the single-colormagenta dots.

However, even in a case where both the threshold matrix for cyan and thethreshold matrix for magenta have blue noise characteristics, thearrangement of overlapping dots of cyan and magenta printed according tothe threshold matrixes does not have blue noise characteristics.Additionally, since such overlapping dots have higher dot powers thanthe single-color cyan dots and the single-color magenta dots,granularity of the overlapping dots may be unfavorably conspicuous inthe highly dispersed cyan dots and magenta dots. That is, even with thetechnique of Japanese Patent Laid-Open No. 2017-38127, an image in whichoverlapping dots with high dot powers are mixed may have conspicuousgranularity.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-described problems. Inthis view, the object of the present invention is to provide an imageprocessing apparatus that enables printing of an image with noconspicuous granularity in a configuration of printing with dots ofmultiple color materials mixed on a printing medium.

In a first aspect of the present invention, there is provided an imageprocessing apparatus, comprising: a gradation data obtainment unitconfigured to obtain first gradation data corresponding to a gradationvalue of a first color and second gradation data corresponding to agradation value of a second color for a processing-target pixel; athreshold obtainment unit configured to obtain a first threshold for theprocessing-target pixel from a first threshold matrix including aplurality of arrayed thresholds for pixels and obtain a second thresholdfor the processing-target pixel from a second threshold matrix in whichthe thresholds for the pixels are arrayed at such pixel positions thatorder of the pixel positions is inverse to order of pixel positions inthe first threshold matrix in a case where the pixel positions arearranged in ascending order of the thresholds; and a generation unitconfigured to generate first quantization data and second quantizationdata that have a smaller number of gradations than the number ofgradations of the first gradation data and the second gradation databased on the first threshold, the second threshold, the first gradationdata, and the second gradation data: the image processing apparatusperforming image processing to print a color material of the first colorbased on the first quantization data and to print a color material ofthe second color based on the second quantization data, wherein thegeneration unit, in a case where a sum of the first gradation data andthe second gradation data is equal to or smaller than the maximum valueof the thresholds arrayed in the first threshold matrix, generates thefirst quantization data based on a result of comparing the firstgradation data with the first threshold and generates the secondquantization data based on a result of comparing the second gradationdata with the second threshold, and in a case where the sum is greaterthan the maximum value, generates first overlapping gradation data andsecond overlapping gradation data by dividing a value that is obtainedby subtracting the maximum value from the sum into two, generates thefirst quantization data based on a result of comparing the firstoverlapping gradation data with the second threshold or a result ofcomparing a difference between the first gradation data and the firstoverlapping gradation data with the first threshold, and generates thesecond quantization data based on a result of comparing the secondoverlapping gradation data with the first threshold or a result ofcomparing a difference between the second gradation data and the secondoverlapping gradation data with the second threshold.

In a second aspect of the present invention, there is provided an imageprocessing apparatus, comprising: a gradation data obtainment unitconfigured to obtain first gradation data corresponding to a gradationvalue of a first color and second gradation data corresponding to agradation value of a second color for a processing-target pixel; athreshold obtainment unit configured to obtain a first threshold for theprocessing-target pixel from a first threshold matrix including aplurality of arrayed thresholds for pixels and obtain a second thresholdfor the processing-target pixel from a second threshold matrix in whichthe thresholds for the pixels are arrayed at such pixel positions thatorder of the pixel positions is inverse to order of pixel positions inthe first threshold matrix in a case where the pixel positions arearranged in ascending order of the thresholds; and a generation unitconfigured to generate first quantization data and second quantizationdata that have a smaller number of gradations than the number ofgradations of the first gradation data and the second gradation databased on the first threshold, the second threshold, the first gradationdata, and the second gradation data: the image processing apparatusperforming image processing to print a color material of the first colorbased on the first quantization data and to print a color material ofthe second color based on the second quantization data, wherein thegeneration unit, in a case where a sum of the first gradation data andthe second gradation data is equal to or smaller than the maximum valueof the thresholds arrayed in the first threshold matrix, generates thefirst quantization data based on a result of comparing the firstgradation data with the first threshold and generates the secondquantization data based on a result of comparing the second gradationdata with the second threshold, and in a case where the sum is greaterthan the maximum value, generates a first correction amount and a secondcorrection amount by dividing a value that is obtained by subtractingthe maximum value from the sum into two, generates the firstquantization data by comparing the first gradation data with a valueobtained by adding the first correction amount to the first thresholdand generates the second quantization data by comparing the secondgradation data with a value obtained by adding the second correctionamount to the second threshold.

In a third aspect of the present invention, there is provided an imageprocessing apparatus, comprising: a gradation data obtainment unitconfigured to obtain first gradation data corresponding to a gradationvalue of a first color and second gradation data corresponding to agradation value of a second color for a processing-target pixel; athreshold obtainment unit configured to obtain a first threshold for theprocessing-target pixel from a first threshold matrix including aplurality of arrayed thresholds for pixels and obtain a second thresholdfor the processing-target pixel from a second threshold matrix in whichthe thresholds for the pixels are arrayed at such pixel positions thatorder of the pixel positions is inverse to order of pixel positions inthe first threshold matrix in a case where the pixel positions arearranged in ascending order of the thresholds; and a generation unitconfigured to generate first quantization data and second quantizationdata that have N gradations (N is an integer equal to or greater than3), with N being smaller than the number of gradations of the firstgradation data and the second gradation data, based on the firstthreshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing apparatus performing imageprocessing to print a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein the generation unitequally divides a gradation domain of the first gradation data and thesecond gradation data into (N−1) ranges to calculate a first intra-rangegradation value that is a gradation value of the first gradation data ina range including the first gradation data and calculate a secondintra-range gradation value that is a gradation value of the secondgradation data in a range including the second gradation data, and in acase where the range including the first gradation data and the rangeincluding the second gradation data are equal, i) in a case where a sumof the first intra-range gradation value and the second intra-rangegradation value is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, generates the firstquantization data based on a result of comparing the first intra-rangegradation value with the first threshold and generates the secondquantization data based on a result of comparing the second intra-rangegradation value with the second threshold, and ii) in a case where thesum is greater than the maximum value, generates first overlappinggradation data and second overlapping gradation data by dividing a valuethat is obtained by subtracting the maximum value from the sum into two,generates the first quantization data based on a result of comparing thefirst overlapping gradation data with the second threshold or a resultof comparing a difference between the first intra-range gradation valueand the first overlapping gradation data with the first threshold, andgenerates the second quantization data based on a result of comparingthe second overlapping gradation data with the first threshold or aresult of comparing a difference between the second intra-rangegradation value and the second overlapping gradation data with thesecond threshold.

In a fourth aspect of the present invention, there is provided an imageprocessing apparatus, comprising: a gradation data obtainment unitconfigured to obtain first gradation data corresponding to a gradationvalue of a first color and second gradation data corresponding to agradation value of a second color for a processing-target pixel; athreshold obtainment unit configured to obtain a first threshold for theprocessing-target pixel from a first threshold matrix including aplurality of arrayed thresholds for pixels and obtain a second thresholdfor the processing-target pixel from a second threshold matrix in whichthe thresholds for the pixels are arrayed at such pixel positions thatorder of the pixel positions is inverse to order of pixel positions inthe first threshold matrix in a case where the pixel positions arearranged in ascending order of the thresholds; and a generation unitconfigured to generate first quantization data and second quantizationdata that have N gradations (N is an integer equal to or greater than3), with N being smaller than the number of gradations of the firstgradation data and the second gradation data, based on the firstthreshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing apparatus performing imageprocessing to print a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein the generation unitequally divides a gradation domain of the first gradation data and thesecond gradation data into (N−1) ranges to calculate a first intra-rangegradation value that is a gradation value of the first gradation data ina range including the first gradation data and calculate a secondintra-range gradation value that is a gradation value of the secondgradation data in a range including the second gradation data, and in acase where the range including the first gradation data and the rangeincluding the second gradation data are equal, i) in a case where a sumof the first intra-range gradation value and the second intra-rangegradation value is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, generates the firstquantization data based on a result of comparing the first intra-rangegradation value with the first threshold and generates the secondquantization data based on a result of comparing the second intra-rangegradation value with the second threshold, and ii) in a case where thesum is greater than the maximum value, generates a first thresholdoffset amount and a second threshold offset amount by dividing a valuethat is obtained by subtracting the maximum value from the sum into two,generates the first quantization data based on a result of comparing thefirst intra-range gradation value with a value obtained by adding thefirst threshold offset amount to the first threshold and generates thesecond quantization data based on a result of comparing the secondintra-range gradation value with a value obtained by adding the secondthreshold offset amount to the second threshold.

In a fifth aspect of the present invention, there is provided an imageprocessing method, comprising: a gradation data obtaining step ofobtaining first gradation data corresponding to a gradation value of afirst color and second gradation data corresponding to a gradation valueof a second color for a processing-target pixel; a threshold obtainingstep of obtaining a first threshold for the processing-target pixel froma first threshold matrix including a plurality of arrayed thresholds forpixels and obtain a second threshold for the processing-target pixelfrom a second threshold matrix in which the thresholds for the pixelsare arrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have a smaller number of gradationsthan the number of gradations of the first gradation data and the secondgradation data based on the first threshold, the second threshold, thefirst gradation data, and the second gradation data: the imageprocessing method performing image processing for printing a colormaterial of the first color based on the first quantization data and toprint a color material of the second color based on the secondquantization data, wherein in the generating step, in a case where a sumof the first gradation data and the second gradation data is equal to orsmaller than the maximum value of the thresholds arrayed in the firstthreshold matrix, the first quantization data is generated based on aresult of comparing the first gradation data with the first thresholdand the second quantization data is generated based on a result ofcomparing the second gradation data with the second threshold, and inthe generating step, in a case where the sum is greater than the maximumvalue, first overlapping gradation data and second overlapping gradationdata are generated by dividing a value that is obtained by subtractingthe maximum value from the sum into two, the first quantization data isgenerated based on a result of comparing the first overlapping gradationdata with the second threshold or a result of comparing a differencebetween the first gradation data and the first overlapping gradationdata with the first threshold, and the second quantization data isgenerated based on a result of comparing the second overlappinggradation data with the first threshold or a result of comparing adifference between the second gradation data and the second overlappinggradation data with the second threshold.

In a sixth aspect of the present invention, there is provided an imageprocessing method, comprising: a gradation data obtaining step ofobtaining first gradation data corresponding to a gradation value of afirst color and second gradation data corresponding to a gradation valueof a second color for a processing-target pixel; a threshold obtainingstep of obtaining a first threshold for the processing-target pixel froma first threshold matrix including a plurality of arrayed thresholds forpixels and obtain a second threshold for the processing-target pixelfrom a second threshold matrix in which the thresholds for the pixelsare arrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have a smaller number of gradationsthan the number of gradations of the first gradation data and the secondgradation data based on the first threshold, the second threshold, thefirst gradation data, and the second gradation data: the imageprocessing method further comprising an image processing step ofprinting a color material of the first color based on the firstquantization data and to print a color material of the second colorbased on the second quantization data, wherein in the generating step,in a case where a sum of the first gradation data and the secondgradation data is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, the first quantizationdata is generated based on a result of comparing the first gradationdata with the first threshold and the second quantization data isgenerated based on a result of comparing the second gradation data withthe second threshold, and in a case where the sum is greater than themaximum value, a first correction amount and a second correction amountare generated by dividing a value that is obtained by subtracting themaximum value from the sum into two, the first quantization data isgenerated by comparing the first gradation data with a value obtained byadding the first correction amount to the first threshold, and thesecond quantization data is generated by comparing the second gradationdata with a value obtained by adding the second correction amount to thesecond threshold.

In a seventh aspect of the present invention, there is provided an imageprocessing method, comprising: a gradation data obtaining step ofobtaining first gradation data corresponding to a gradation value of afirst color and second gradation data corresponding to a gradation valueof a second color for a processing-target pixel; a threshold obtainingstep of obtaining a first threshold for the processing-target pixel froma first threshold matrix including a plurality of arrayed thresholds forpixels and obtain a second threshold for the processing-target pixelfrom a second threshold matrix in which the thresholds for the pixelsare arrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have N gradations (N is an integerequal to or greater than 3), with N being smaller than the number ofgradations of the first gradation data and the second gradation data,based on the first threshold, the second threshold, the first gradationdata, and the second gradation data: the image processing methodperforming image processing for printing a color material of the firstcolor based on the first quantization data and to print a color materialof the second color based on the second quantization data, wherein inthe generating step, a gradation domain of the first gradation data andthe second gradation data is equally divided into (N−1) ranges tocalculate a first intra-range gradation value that is a gradation valueof the first gradation data in a range including the first gradationdata and calculate a second intra-range gradation value that is agradation value of the second gradation data in a range including thesecond gradation data, and in a case where the range including the firstgradation data and the range including the second gradation data areequal, i) in a case where a sum of the first intra-range gradation valueand the second intra-range gradation value is equal to or smaller thanthe maximum value of the thresholds arrayed in the first thresholdmatrix, the first quantization data is generated based on a result ofcomparing the first intra-range gradation value with the first thresholdand the second quantization data is generated based on a result ofcomparing the second intra-range gradation value with the secondthreshold, and ii) in a case where the sum is greater than the maximumvalue, first overlapping gradation data and second overlapping gradationdata are generated by dividing a value that is obtained by subtractingthe maximum value from the sum into two, the first quantization data isgenerated based on a result of comparing the first overlapping gradationdata with the second threshold or a result of comparing a differencebetween the first intra-range gradation value and the first overlappinggradation data with the first threshold, and the second quantizationdata is generated based on a result of comparing the second overlappinggradation data with the first threshold or a result of comparing adifference between the second intra-range gradation value and the secondoverlapping gradation data with the second threshold.

In a eighth aspect of the present invention, there is provided an imageprocessing method, comprising: a gradation data obtaining step ofobtaining first gradation data corresponding to a gradation value of afirst color and second gradation data corresponding to a gradation valueof a second color for a processing-target pixel; a threshold obtainingstep of obtaining a first threshold for the processing-target pixel froma first threshold matrix including a plurality of arrayed thresholds forpixels and obtain a second threshold for the processing-target pixelfrom a second threshold matrix in which the thresholds for the pixelsare arrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have N gradations (N is an integerequal to or greater than 3), with N being smaller than the number ofgradations of the first gradation data and the second gradation data,based on the first threshold, the second threshold, the first gradationdata, and the second gradation data: the image processing methodperforming image processing for printing a color material of the firstcolor based on the first quantization data and to print a color materialof the second color based on the second quantization data, wherein inthe generating step, a gradation domain of the first gradation data andthe second gradation data is equally divided into (N−1) ranges tocalculate a first intra-range gradation value that is a gradation valueof the first gradation data in a range including the first gradationdata and calculate a second intra-range gradation value that is agradation value of the second gradation data in a range including thesecond gradation data, and in a case where the range including the firstgradation data and the range including the second gradation data areequal, i) in a case where a sum of the first intra-range gradation valueand the second intra-range gradation value is equal to or smaller thanthe maximum value of the thresholds arrayed in the first thresholdmatrix, the first quantization data is generated based on a result ofcomparing the first intra-range gradation value with the first thresholdand the second quantization data is generated based on a result ofcomparing the second intra-range gradation value with the secondthreshold, and ii) in a case where the sum is greater than the maximumvalue, a first threshold offset amount and a second threshold offsetamount are generated by dividing a value that is obtained by subtractingthe maximum value from the sum into two, the first quantization data isgenerated based on a result of comparing the first intra-range gradationvalue with a value obtained by adding the first threshold offset amountto the first threshold, and the second quantization data is generatedbased on a result of comparing the second intra-range gradation valuewith a value obtained by adding the second threshold offset amount tothe second threshold.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that shows a configuration for controlling aninkjet printing system;

FIG. 2 is a schematic perspective view of a printing apparatus;

FIG. 3 is a flowchart for describing image processing of a firstembodiment;

FIG. 4 is a block diagram of quantization processing of the firstembodiment;

FIG. 5 is a diagram that shows a dot printing state in a case ofperforming “sequentially adding type inter-color processing;”

FIG. 6A is a diagram that shows a dot arrangement state, and FIG. 6B isa diagram that shows spatial frequency properties;

FIG. 7 is a diagram that shows a threshold matrix for a first color anda threshold matrix for a second color;

FIGS. 8A and 8B are diagrams that show dot printing states correspondingto thresholds;

FIGS. 9A and 9B are comparative diagrams of a case of using differentthreshold matrixes;

FIGS. 10A and 10B are comparative diagrams of the case of usingdifferent threshold matrixes;

FIGS. 11A to 11C are flowcharts of the quantization processing of thefirst embodiment;

FIGS. 12A and 12B are diagrams that show dot arrangement states in“low-gradation processing;”

FIGS. 13A and 13B are diagrams that show dot arrangement states in“high-gradation processing;”

FIGS. 14A and 14B are diagrams for comparing dot printing states;

FIGS. 15A and 15B are diagrams for comparing dot printing states;

FIGS. 16A and 16B are diagrams for comparing dot printing states;

FIG. 17 is a diagram for comparing power distributions of spatialfrequencies;

FIG. 18 is a flowchart for describing image processing of a secondembodiment;

FIG. 19 is a block diagram of quantization processing of the secondembodiment;

FIG. 20 is a schematic diagram for describing a concept of thequantization processing of the second embodiment;

FIGS. 21A to 21C are diagrams for describing setting examples ofcomparative values;

FIG. 22 is a diagram that shows correspondence between quantizationreference values and comparative values in the second embodiment;

FIG. 23 is a flowchart of quantization processing of a first color ofthe second embodiment;

FIG. 24 is a flowchart of quantization processing of a second color ofthe second embodiment;

FIG. 25 is a diagram that shows correspondence between quantizationreference values and comparative values in a third embodiment;

FIG. 26 is a block diagram of quantization processing of a fourthembodiment;

FIG. 27 is a flowchart of the quantization processing of the fourthembodiment;

FIGS. 28A to 28C are diagrams that show a threshold matrix having bluenoise characteristics; and

FIGS. 29A and 29B are diagrams that show blue noise characteristics andvisual properties of human.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

FIG. 1 is a block diagram that shows a configuration for controlling aninkjet printing system applicable to the present invention. An inkjetprinting system of this embodiment includes an image supply device 3, animage processing apparatus 2, and an inkjet printing apparatus 1(hereinafter, also called a printing apparatus, simply). Predeterminedimage processing is performed by the image processing apparatus 2 onimage data supplied from the image supply device 3, and the image datais then transmitted to the printing apparatus 1 and is printed with inksas color materials.

In the printing apparatus 1, a printing apparatus main control unit 101is for controlling the overall printing apparatus 1 and includes a CPU,ROM, RAM, and so on. A printing buffer 102 can store image data beforetransference of the image data to a printing head 103 as raster data.The printing head 103 is an inkjet printing head including multipleprinting elements capable of ejecting the inks as droplets, and theprinting head 103 ejects the inks from the corresponding printingelements based on the image data stored in the printing buffer 102. Inthis embodiment, rows of the printing elements of three colors, whichare cyan, magenta, and yellow, are aligned on the printing head 103.

A feed and delivery motor control unit 104 conveys a printing medium andcontrols feeding and delivering of the printing medium. A printingapparatus interface (I/F) 105 exchanges data signals with the imageprocessing apparatus 2. An I/F signal line 114 connects the printingapparatus interface (I/F) 105 and the image processing apparatus 2. TheI/F signal line 114 may be a signal line in compliance with thespecifications of Centronics, for example. A data buffer 106 temporarilystores image data received from the image processing apparatus 2. Asystem bus 107 connects the functions of the printing apparatus 1 witheach other.

Meanwhile, in the image processing apparatus 2, an image processingapparatus main control unit 108 is for performing many kinds ofprocessing on the image supplied from the image supply device 3 togenerate image data that can be printed by the printing apparatus 1, andincludes a CPU, ROM, RAM, and so on. The image processing apparatus maincontrol unit 108 also includes the later-described characteristicconfiguration of the present invention shown in FIGS. 4, 19, and 26, andthe flowcharts described with reference to FIGS. 3, 11A to 11C, 18, 23,24, and 27 show processes executed by the CPU in the image processingapparatus main control unit 108. An image processing apparatus interface(I/F) 109 exchanges data signals with the printing apparatus 1. Anexternal connection interface (I/F) 113 exchanges image data and thelike with the externally connected image supply device 3. A display unit110 displays various kinds of information to the user, and an LCD may beapplied as the display unit 110, for example. An operation unit 111 is amechanism that allows the user to perform command operations, and akeyboard and mouse may be applied as the operation unit 111, forexample. A system bus 112 connects the image processing apparatus maincontrol unit 108 with the functions.

In addition to the image data supplied from the image processingapparatus 2, the printing apparatus 1 may directly receive and printimage data stored in a storage medium such as a memory card and imagedata from a digital camera.

FIG. 2 is a schematic perspective view of the printing apparatus 1 usedin this embodiment. The printing apparatus 1 has a function of a normalPC printer that receives and prints the data from the image processingapparatus 2 and a function of printing the image data stored in thestorage medium such as a memory card and the image data received from adigital camera.

A main body forming an outer shell of the printing medium 1 includesexterior members, which are a lower case 1001, an upper case 1002, anaccess cover 1003, a feed tray 1007, and a delivery tray 1004. The lowercase 1001 forms a substantially lower half portion of the apparatus 1,and the upper case 1002 forms a substantially upper half portion of themain body. The combination of the two cases forms a housing space inwhich the mechanisms are housed.

The feed tray 1007 can stack and hold multiple printing media and isadapted to automatically feed the top one of the printing medium to theinside of the apparatus once a printing command is inputted. On theother hand, the delivery tray 1004 includes one end portion that ispivotably held by the lower case 1001, and the pivot movement allows anopening formed at a front surface portion of the lower case 1001 to openand close. To execute a printing operation, the delivery tray 1004 ispivoted toward the front surface to allow the opening to open and close,and this makes it possible to deliver the printing sheets from theopening and to sequentially stack the delivered printing sheets. Thedelivery tray 1004 stores two auxiliary trays 1004 a and 1004 b, and anarea for supporting the printing medium can be enlarged and reduced inthree levels by pulling the trays as needed.

In the inside space of the apparatus, mechanisms such as the printinghead 103 for printing an image on the printing medium, a carriage thatis mounted with the printing head 103 and an ink tank and movable to anX direction indicated in FIG. 2, and a transportation mechanism fortransporting a predetermined amount of the printing medium in the Ydirection, and the like are arranged.

Once the printing command is inputted, the printing medium conveyedinside the apparatus from the feed tray 1007 is transported to aprintable area by the printing head 103. Then, after printing scan isperformed once by the printing head 103, the conveying mechanism conveysthe printing medium in the Y direction corresponding to a distance of aprinting width D. An image is gradually formed on the printing medium byrepeating the above-described printing scan by the printing head 103 andthe conveying of the printing medium. Once the printing is completed,the printing medium is delivered on the delivery tray 1004. Although aserial inkjet printing apparatus is described as an example herein, theprinting apparatus may be a full-line inkjet printing apparatus.

The access cover 1003 includes one end portion that is pivotably held bythe upper case 1002 to allow an opening formed at an upper surface ofthe printing apparatus 1 to open and close. The printing head 103, theink tank, and the like stored inside the main body can be replaced byopening the access cover 1003. Although illustration is omitted, aprotrusion is formed on a back surface of the access cover 1003 to bedetected by a microswitch on the main body while closing the accesscover 1003. That is, it is possible to detect an open/close state of theaccess cover 1003 based on a detection result of the protrusion obtainedby the microswitch.

A power key 1005 to be pressed is provided on an upper surface of theupper case 1002. An operation panel 1010 including a liquid crystaldisplay unit 110, various key switches, and the like is also provided onthe upper surface of the upper case 1002.

A distance selection lever 1008 is a lever for adjusting a distancebetween an ink discharge surface of the printing head 103 and a surfaceof the printing medium. A card slot 1009 is an opening for receiving anadaptor to which a memory card can be inserted. Image data stored in thememory card is transmitted to a control unit 3000 of the printingapparatus through the adaptor inserted to the card slot 1009, and afterpredetermined processing is performed on the image data, the image datais printed on the printing medium. The memory card (PC) may be a compactflash (registered trademark) memory, smart media, memory stick, and thelike, for example. A viewer (liquid crystal display unit) 1011 displaysa single image or an index image during, for example, searching for animage to be printed from the images stored in the memory card. In thisembodiment, the viewer 1011 is detachably attached on the main body ofthe printing apparatus 1. A terminal 1012 is a terminal for connecting adigital camera, and a terminal 1013 is a USB bus connector forconnecting a personal computer (PC).

FIG. 3 is a flowchart for describing the image processing performed bythe image processing apparatus main control unit 108 of this embodiment.This processing is executed by the CPU in the image processing apparatusmain control unit 108 according to a program stored in the ROM.

In FIG. 3, once image data of processing-target pixels is inputted fromthe image supply device 3 (step S200), first, in step S201, the imageprocessing apparatus main control unit 108 executes color correction.The image data received by the image processing apparatus 2 from theimage supply device 3 is 8-bit lightness data of R (red), G (green), andB (blue) for expressing a standardized color space such as sRGB. In stepS201, the lightness data is converted to RGB 12-bit lightness datacorresponding to a color space unique to the printing apparatus. Toconvert the signal values, a publicly known method such as referring toa lookup table (LUT) stored in the ROM or the like in advance may beemployed.

In step S202, the image processing apparatus main control unit 108separates the converted RGB data into pieces of 12-bit gradation data(density data) of C (cyan), M (magenta), and Y (yellow), which are inkcolors of the printing apparatus. In this stage, three channels (threecolors) of 12-bit gray images are generated. In the ink color separationprocessing, it is also possible to refer to the lookup table (LUT)stored in the ROM or the like in advance like the color correctionprocessing.

In step S203, the image processing apparatus main control unit 108performs predetermined quantization processing on the pieces of 12-bitgradation data corresponding to the ink colors and converts the 12-bitgradation data to 1-bit binary data. The quantization processing isdescribed in detail later.

Once the quantization processing ends, the image processing apparatusmain control unit 108 outputs the generated binary data to the printingapparatus (step S205). With that, the processing ends.

While the steps described in FIG. 3 are processed by the inkjet printingsystem of this embodiment, a boundary of steps processed by the imageprocessing apparatus 2 and steps processed by the printing apparatus 1is not precisely determined. For example, in a case where the steps fromthe beginning to the quantization are performed by the image processingapparatus 2, the quantized data may be transferred to the printingapparatus 1, and the printing operation may be controlled by theprinting apparatus main control unit 101 that performs index expansionin step S204 using an index pattern stored in the data buffer 106.Depending on the capability, the printing apparatus 1 may directlyreceive multi-valued RGB image data and perform all the steps of S201 toS203.

FIG. 4 is a block diagram of the quantization processing executed instep S203 in FIG. 3. In the quantization processing of this embodiment,first, processing on the input values is performed, processing on thethresholds is then performed, and quantization processing by a dithermethod is finally performed. The series of processes are parallelprocessed on the different colors (channels). Hereinafter, the processesare described in detail with reference to FIG. 4.

A gradation data obtainment unit 301 obtains the pieces of 12-bitgradation data indicating density of the pixels. The gradation dataobtainment unit 301 of this embodiment can receive signals of up to 12bits of eight colors. FIG. 4 shows a state in which pieces of 12-bitdata of a first color to a third color are inputted.

A noise addition processing unit 302 adds predetermined noise to the12-bit gradation data. Adding of noise makes it possible to avoidsequential arrangement of the same patterns even in a case where piecesof gradation data of the same level are sequentially inputted, and thisinhibits stripe, texture, and the like. The noise addition processingunit 302 multiplies a predetermined random table, fixed intensity, andvariable intensity corresponding to an input value, and thus noise isgenerated for each pixel and added to the input value. Note that therandom table is a table adapted to set the polarity of noise, and sets aplus, zero, or a minus for each pixel position. The random table in thisembodiment can have at most eight faces, and the size of each table canbe arbitrarily set. The fixed intensity indicates the intensity of noiseamount, and the magnitude of the intensity determines whether noise islarge or small. In this embodiment, a noise amount can be appropriatelyadjusted by setting a random table and fixed intensity optimum for eachprint mode depending on the granularity and the degrees of stripe andtexture of an image.

The above-described processes by the gradation data obtainment unit 301and the noise addition processing unit 302 are parallel processed on thepieces of gradation data of different colors. That is, in thisembodiment, pieces of 12-bit data are respectively generated for cyan,magenta, and yellow and are inputted to a dither processing unit 311.

In the dither processing unit 311, the pieces of 12-bit data of thedifferent colors are inputted to an inter-color processing unit 304. Theinter-color processing unit 304 performs predetermined processing on theinputted gradation data of a processing-target color based on thegradation data of the processing-target color and the gradation data ofa color other than the processing-target color to obtaincomparison-target data, and transmits the comparison-target data to aquantization processing unit 306. The quantization processing unit 306compares the received comparison-target data with a threshold obtainedfrom a threshold obtainment unit 305 and determines printing (1) ornot-printing (0) (high level or low level) of the processing-targetcolor.

The threshold obtainment unit 305 selects one corresponding thresholdmatrix from multiple dither patterns 310 stored in a memory such as theROM and obtains a threshold for a pixel position of theprocessing-target data. In this embodiment, the dither pattern 310 is athreshold matrix including arrayed thresholds 0 to 4095 and having bluenoise characteristics, and the size and shape of the dither pattern 310may have variations such as 512×512 pixel, 256×256 pixel, and 512×256pixel. That is, the memory stores such multiple threshold matrixes indifferent sizes and shapes in advance, and the threshold obtainment unit305 selects a threshold matrix corresponding to the print mode and theink color from the threshold matrixes in the memory. Then, the thresholdobtainment unit 305 provides the inter-color processing unit 304 with athreshold for a pixel position (i) of the processing-target data amongmultiple thresholds arrayed in the selected threshold matrix.

In this process, in a case of employing the later-described“sequentially adding type inter-color processing” of this embodiment,the inter-color processing unit 304 adds a density value to the receivedgradation data of the processing-target color, the density value beingindicated by gradation data of a color that is other than theprocessing-target color and is set as a reference color in advance.Then, the inter-color processing unit 304 transmits the obtained valueto the quantization processing unit 306 as the comparison-target data.If the value obtained by the addition is greater than the maximum valueof the threshold matrix (that is 4096 in the case of 12 bits), themaximum value is subtracted from the obtained value, and the newlyobtained value is transmitted to the quantization processing unit 306.

In this process, “no” reference color is set for a color set as thefirst color in the “sequentially adding type inter-color processing.”Thus, for the first color, the quantization processing unit 306 performsquantization processing according to the threshold matrix stored as thedither pattern 310. On the other hand, “first color” is set as thereference color for a color set as the second color in the “sequentiallyadding type inter-color processing.” Thus, for the second color,quantization processing is performed based on an input gradation valueIn2 of the second color, an input gradation value In1 of the firstcolor, and a threshold Dth stored in the threshold matrix.

Specifically, first, a value In′2 is obtained by adding the inputgradation value In1 of the first color to the input gradation value In2of the second color:

In′2=In1+In2.

If the value In′2 obtained by the addition is greater than the maximumvalue Dthmax among the thresholds, the maximum value Dthmax issubtracted:

In′2=In′2−Dthmax.

Thereafter, in a case where In′2>Dth and In1<Dth, printing (1) is setfor the second color in the processing-target image.

On the other hand, in a case where In′2>Dth and In1>Dth, or in a casewhere In′2≤Dth, not-printing (0) is set for the second color in theprocessing-target image. Hereinafter, the above-described processing iscontinuously called the “sequentially adding type inter-colorprocessing.”

FIG. 5 is a diagram that shows a dot printing state with respect tothresholds stored in the threshold matrix in a case of the quantizationwith the “sequentially adding type inter-color processing.” Thehorizontal axis indicates a density value, and 0% corresponds to theminimum value “0” among the thresholds arrayed in the threshold matrixwhile 100% corresponds to the maximum value among those thresholds (thatis 4095 in the case of 12 bits). Now, there is shown a case wheredensity data of around 20% (820) is inputted for each of the first colorand the second color.

In the “sequentially adding type inter-color processing,” dots of thefirst color for which “no” reference color is set are printed inpositions in which thresholds 0 to 20% (0 to 820) are set. On the otherhand, dots of the second color for which “first color” is set as thereference color are printed in positions in which thresholds 20 to 40%(821 to 1639) are set. Additionally, mixed dots of the first color andthe second color are printed in positions in which thresholds 0 to 40%(0 to 1639) are set. Dots of the first color and dots of the secondcolor are not overlapped with each other since they are printed indifferent pixel positions in the threshold matrix.

FIG. 6A and FIG. 6B are diagrams that respectively show a dotarrangement state and power distributions of spatial frequencies in acase of performing the “sequentially adding type inter-color processing”described in FIG. 5 using the threshold matrix having blue noisecharacteristics. As shown in FIG. 6A, all of the single-color dots ofthe first color, the single-color dots of the second color, and themixed dots of the two colors are arranged with high dispersibility. Asshown in FIG. 6B, all types of dots are arranged with blue noisecharacteristics that allow for suppressed low frequency components,rapid rise, and flat high frequency components.

However, blue noise characteristics of the single-color dots of thesecond color are slightly different from those of the single-color dotsof the first color and the mixed dots, since the single-color dots ofthe second color are arranged in the positions of middle thresholds ofthe threshold matrix (that are thresholds not from 0 but between 821 and1640). That is, the power spectrum rises early, low frequency componentsbecomes high, and the dot dispersibility is low.

According to examination by the inventors, it is confirmed that it iseffective to use two threshold matrixes in which orders of positions ofthe pixels corresponding to orders of thresholds are inverse to eachother. Specifically, in a case where Dthmax is the maximum value amongthresholds and Dth is a threshold in a predetermined pixel position in athreshold matrix having blue noise characteristics, a threshold Dth1 ofthe first color and a threshold Dth2 of the second color in apredetermined pixel position may be determined by the followingexpressions:

Dth1=Dth Dth2=Dthmax−Dth  (Expression 2).

FIG. 7 is a diagram for describing a threshold matrix for the firstcolor and a threshold matrix for the second color created by theabove-described method. In this case, for the sake of simplicity, thethreshold matrixes each have 4×4 pixel area in which thresholds 0 to 15are set, and Dthmax=15. For a pixel in which the minimum value 0 is setin the first threshold matrix, the maximum value 15 is set in the secondthreshold matrix, and for a pixel in which the maximum value 15 is setin the first threshold matrix, the minimum value 0 is set in the secondthreshold matrix.

The coordinate positions in FIG. 7 use alphabet signs. Specifically, thecoordinate positions of the pixels in the first threshold matrix and thesecond threshold matrix in FIG. 7 are indicated by the alphabet signs.In a case where the signs at the coordinate positions of the thresholdsin the first matrix are put in ascending order of the correspondingthresholds, the order of the coordinate positions is AHJOGPBIDKMFNCEL.On the other hand, in a case where the signs at the coordinate positionsof the thresholds in the second matrix are put in ascending order of thecorresponding thresholds, the order of the coordinates isLECNFMKDIBPGOJHA. That is, the first threshold matrix and the secondthreshold matrix have a relationship of inverse sign order in the casewhere the signs at the coordinate positions of the thresholds are put inascending order of the thresholds or in order of the number incrementedfrom 0 by 1. Needless to say, the inverse relationship is also made in acase of putting the signs at the coordinate positions in descendingorder of the thresholds.

Hereinafter, the two threshold matrixes having the inverse orders ofpixel positions according to the order of thresholds are called“threshold matrixes in the inverse relationship.” The quantizationprocessing performed on two colors using thresholds in the inverserelationship according to (Expression 2) based on one threshold matrixis called “quantization processing of Expression (2).”

FIGS. 8A and 8B are diagrams that show dot printing states correspondingto thresholds in a case of performing the quantization processing on thefirst color and the second color with the first threshold matrix and thesecond threshold matrix in the inverse relationship. In this case,density data of around 25% (1024) is inputted to each of the first colorand second color.

FIG. 8A shows a printing state of the first color dots corresponding tothresholds in the first threshold matrix and a printing state of thesecond color dots corresponding to thresholds in the second thresholdmatrix. FIG. 8B shows printing states of the first color dots and thesecond color dots corresponding to the thresholds in the first thresholdmatrix.

According to examination by the inventors, for quantization processingon the first color and the second color both having high dot powers, itis confirmed that the granularity of the second color and the entireimage is less conspicuous by using the threshold matrixes in the inverserelationship than the case of applying the “sequentially adding typeinter-color processing.” In other words, dots printed in a sequentialthreshold area including the minimum value (the first color in FIG. 8B)and dots printed in a sequential threshold area including the maximumvalue (the second color in FIG. 8B) can have higher dispersibility thandots printed in a sequential threshold area including neither theminimum nor maximum values (the second color in FIG. 5).

Additionally, use of the threshold matrixes in the inverse relationshipcan minimize appearance of overlapping dots of the first and secondcolors. FIGS. 9A and 9B are diagrams for comparing a case of using twothreshold matrixes with no relationship with a case of using twothreshold matrixes in the inverse relationship for the quantizationprocessing on the first color and the second color. In the case of usingtwo threshold matrixes with no relationship, as shown in FIG. 9A, highdispersibility can be obtained in each case of the first color and thesecond color, but if the two colors are overlapped, overlapping dots ofthe two colors are likely to appear even in a low-gradation area.

In contrast, in the case of using two threshold matrixes in the inverserelationship, as shown in FIG. 9B, high dispersibility can be obtainednot only in each case of the first color and the second color but alsoin a case where the two colors are overlapped, and no overlapping dotsof the two colors appear in a low-gradation area. This is because thepositions in which dots of the first color and the second color arearranged in mutually exclusive threshold areas in the same thresholdmatrix as shown in FIG. 8B.

However, even in the case of using two threshold matrixes in the inverserelationship, overlapping dots of the first and second colors appear ina gradation area in which the gradation value of the first color and thegradation value of the second color increase and their sum becomesgreater than the maximum threshold. In such a gradation area, sometimesthe granularity is more conspicuous in the case of using two thresholdmatrixes in the inverse relationship than the case of performing the“sequentially adding type inter-color processing” with a singlethreshold matrix.

FIGS. 10A and 10B are diagrams for comparing dot printing statescorresponding to thresholds in the case of performing the “sequentiallyadding type inter-color processing” with a single threshold matrix andthe case of performing the “quantization processing of Expression (2).”Both the diagrams show a case where density data of around 75% (3072) isinputted to each of the first color and the second color.

Note that, for the first color in both the cases, dots are printed inpositions in which thresholds 0 to 75% (0 to 3071) are set.

In contrast, for the second color, dots are printed in areas of 75 to100% (3072 to 4095) and 0 to 50% (0 to 2047) in the case of performingthe “sequentially adding type inter-color processing” as shown in FIG.10A, and dots are printed in an area of 25 to 100% (1024 to 4095) in thecase of performing the “quantization processing of Expression (2)” asshown in FIG. 10B.

Focusing on a threshold area in which both of the first color and secondcolor are printed (a threshold area in which overlapping dots areprinted), it is the area of 0 to 50% (0 to 2047) in FIG. 10A, and it isthe area of 25 to 75% (1024 to 3071) in FIG. 10B. Hereinafter, such athreshold area in which both of the first color and the second color areprinted is called an overlapping gradation area. That is, an overlappinggradation area in the case of the “sequentially adding type inter-colorprocessing” is the sequential area including the minimum threshold, butan overlapping gradation area in the case of the “quantizationprocessing of Expression (2)” is the sequential area including neitherthe maximum nor minimum thresholds. Thus, the dispersibility of theoverlapping dots appearing in the “quantization processing of Expression(2)” is lower than the dispersibility of the overlapping dots appearingin the “sequentially adding type inter-color processing,” and thiscauses the granularity to be conspicuous.

That is, comparing the “sequentially adding type inter-color processing”with the “quantization processing of Expression (2),” the “quantizationprocessing of Expression (2)” achieves better dispersibility in agradation area with no overlapping dots, but the “sequentially addingtype inter-color processing” achieves better dispersibility in agradation area with overlapping dots having high dot power appear.

Based on the above, the inventors found out that it is effective to usedifferent quantization methods between a low-gradation area in which thesum of the first color gradation value and the second color gradationvalue is equal to or smaller than the maximum threshold and ahigh-gradation area in which the sum is greater than the maximum valuein order to obtain the preferable dispersibility in the entire gradationarea.

FIGS. 11A to 11C are flowcharts for describing processes for thequantization of the first color and the second color executed by thedither processing unit 311 of this embodiment. Once gradation data In1of the first color and gradation data In2 of the second color areinputted for a processing-target pixel, first, in S01 in FIG. 11A, thedither processing unit 311 determines whether the sum of the gradationdata In1 of the first color and the gradation data In2 of the secondcolor is greater than 100%. For example, in a case of 12-bit gradationdata, the dither processing unit 311 determines whether the sum of thegradation data In1 and the gradation data In2 is equal to or greaterthan 4096. If the sum is equal to or smaller than 100% (equal to orsmaller than 4095), the process proceeds to “low-gradation processing”S02, and if the sum is greater than 100% (equal to or greater than4096), the process proceeds to “high-gradation processing” S03.

FIG. 11B is a diagram that shows steps of the “low-gradation processing”executed in S02. In the “low-gradation processing” S02 to which theprocess proceeds if the sum of In1 and In2 is equal to or smaller than100%, no overlapping gradation area appears even with the “quantizationprocessing of Expression (2).” Thus, the “quantization processing ofExpression (2)” is performed to give priority to the dispersibility ofsingle-color dots of the first and second colors.

Once the processing is started, first, in S10, the dither processingunit 311 obtains the thresholds Dth1 and Dth2 for a processing-targetpixel. The threshold Dth2 may be read from a second threshold matrixthat is stored in advance as a threshold matrix having the inverserelationship with the first threshold matrix, or may be calculated basedon the threshold Dth1 by using (Expression 2).

Next, in S11, the dither processing unit 311 compares the gradation dataIn1 of the first color with the threshold Dth1 obtained in S10. IfIn1>Dth1, the process proceeds to S15 and an output value (quantizationvalue) of the first color is set to “1” (printing), and if In1≤Dth1, theprocess proceeds to S12.

In S12, the dither processing unit 311 compares the gradation data In2of the second color with the threshold Dth2 obtained in S10. IfIn2>Dth2, the process proceeds to S14 and an output value (quantizationvalue) of the second color is set to “1” (printing), and if In2<Dth2,the process proceeds to S13 and the output values (quantization values)for the first color and the second color are both set to “0”(not-printing).

FIG. 11C is a diagram that shows steps of the “high-gradationprocessing” executed in S03 in FIG. 11A. In the “high-gradationprocessing” S03 to which the process proceeds if the sum of In1 and In2is greater than 100%, an overlapping gradation area appears with the“quantization processing of Expression (2).” Thus, processing that cangive priority to not only the dispersibility of single-color dots of thefirst and second colors but also the dispersibility of overlapping dotsof the first and second colors.

Once the processing is started, first, in S20, the dither processingunit 311 obtains the threshold Dth1 for the first color and thethreshold Dth2 for the second color for a processing-target pixel. Likethe case of the “low-gradation processing,” the threshold Dth2 may beread from a second threshold matrix that is stored in advance, or may becalculated based on the threshold Dth1 by using (Expression 2).

In S21, the dither processing unit 311 obtains an overlapping gradationarea width In3. The overlapping gradation area width In3 indicates awidth of the threshold area in which both of the first color and thesecond color are printed and is expressed by (Expression 3):

In3=In1+In2−Dthmax  (Expression 3).

In S22, the dither processing unit 311 equally divide the overlappinggradation area width In3 into two to calculate a first overlappinggradation width In4 (first overlapping gradation data):

In4=In3/2.

If the thus-obtained value includes a fraction, it is cut off to obtainIn4.

Additionally, in S23, the dither processing unit 311 subtracts the firstoverlapping gradation width In4 from the overlapping gradation areawidth In3 to calculate a second overlapping gradation width In5 (secondoverlapping gradation data):

In5=In3−In4.

In S24, the dither processing unit 311 determines whether In5>Dth1(Condition 1) or In4>Dth2 (Condition 2) is satisfied. If either ofCondition 1 and Condition 2 is satisfied, the process proceeds to S25and the output value (Out1) of the first color and the output value(Out2) of the second color are both set to “1” (printing). The pixelsatisfying Condition 1 is a pixel in which printing is made according tothe first threshold matrix, and the pixel satisfying Condition 2 is apixel in which printing is made according to the second thresholdmatrix.

On the other hand, in S24, if neither Condition 1 nor Condition 2 issatisfied, the process proceeds to S26 and the dither processing unit311 determines whether In1−In4>Dth1 (Condition 3) is satisfied. If(Condition 3) is satisfied, the process proceeds to S27 and the outputvalue (quantization value) of the first color is set to “1” (printing)while the output value (quantization value) of the second color is setto “0” (not-printing). On the other hand, if (Condition 3) is notsatisfied in S26, the process proceeds to S28 and the output value(quantization value) of the first color is set to “0” (not-printing)while the output value (quantization value) of the second color is setto “1” (printing). The pixel satisfying Condition 3 is a pixel in whicha single-color dot of the first color is printed according to the firstthreshold matrix, and the pixel not satisfying Condition 3 is a pixel inwhich a single-color dot of the second color is printed according to thesecond threshold matrix. With that, the processing ends.

Note that, although the pixel satisfying Condition 3 is designated as apixel for printing a single-color dot of the first color and the pixelnot satisfying Condition 3 is designated as a pixel for printing asingle-color dot of the second color in S26, it is possible to apply theopposite condition of Condition 3 in S26. That is, it is possible tomake determination of whether In2−In5>Dth2 (Condition 3′) is satisfied,and if it is satisfied, the first color is set to “0” while the secondcolor is set to “1,” and if it is not satisfied, the first color is setto “1” while the second color is set to “0.”

FIGS. 12A and 12B are diagrams that show dot arrangement states of thefirst color and the second color corresponding to thresholds in the“low-gradation processing.” There is shown a case where the twothreshold matrixes in the inverse relationship shown in FIG. 7 are usedas the first threshold matrix and the second threshold matrix and theinput gradation values are In1=6 and In2=4 (In1+In2=10<15).

Dots of the first color are printed in 0 to 5 in the first thresholdmatrix, and dots of the second color are printed in 0 to 3 (that are 12to 15 in the first threshold matrix) in the second threshold matrix.That is, the single-color dots of the first color are printed in thesequential area including the minimum value of the first thresholdmatrix while the single-color dots of the second color are printed inthe sequential area including the maximum value of the first thresholdmatrix, and thus no overlapping gradation area appears. Consequently,both of the single-color dots of the first color and the single-colordots of the second color are printed with high dispersibility, and asmooth image with no conspicuous granularity can be outputted.

On the other hand, FIGS. 13A and 13B are diagrams that show dotarrangement states of the first color and the second color correspondingto thresholds in the “high-gradation processing.” There is shown a casewhere the two threshold matrixes in the inverse relationship shown inFIG. 7 are used as the first threshold matrix and the second thresholdmatrix and the input gradation values are In1=9 and In2=12(In1+In2=21>15).

Dots of the first color are separately printed in 0 to 6 and 14 to 15 inthe first threshold matrix. Dots of the second color are separatelyprinted in 0 to 8 (that are 7 to 15 in the first threshold matrix) and13 to 15 (that are 0 to 2 in the first threshold matrix) in the secondthreshold matrix. Since the sum of In1 and In2 is greater than 100%,overlapping dots appear and they are separately printed in 0 to 2 and 14to 15 in the first threshold matrix.

That is, overlapping dots having higher dot power than single-color dotsof the first color and single-color dots of the second color areseparately printed in a sequential area including the minimum value (0to 2) and a sequential area including the maximum value (14 to 15).Consequently, all kinds of dots are arranged with high dispersibility,and a smooth image with no conspicuous granularity can be outputted.

FIGS. 14A and 14B are diagrams for comparing dot printing statescorresponding to thresholds in a case where the first gradation data In1and the second gradation data In2 are varied in various ways. FIG. 14Ashows a case of performing the “quantization processing of Expression(2),” and FIG. 14B shows a case of performing the quantizationprocessing of this embodiment.

Both the diagrams show a case where combinations of the first inputgradation value In1 and the second input gradation value In2 (In1, In2)are (25%, 100%) (50%, 75%) (75%, 50%) and (100%, 25%). That is, thereare shown four cases where the overlapping area width In3 is 25%.

In the case of the “quantization processing of Expression (2)” shown inFIG. 14A, positions of the threshold area (pixel positions) in which theoverlapping first color and second color are printed are displacedwithin the threshold matrix depending on the combination of the firstinput gradation value In1 and the second input gradation value In2. Thatis, the threshold area in which the overlapping first color and secondcolor are printed is varied and not fixed in a sequential area includingthe minimum or maximum threshold, and a position in which theoverlapping dot is actually printed is accordingly moved. Consequently,in the case of the “quantization processing of Expression (2),” it maybe difficult to maintain the high dispersibility, and the granularitymay become conspicuous.

In contrast, in the case of the quantization processing of thisembodiment shown in FIG. 14B, the pixel positions (gradation area) inwhich the overlapping first color and second color are printed arestable within the threshold matrix even in the case where the firstinput gradation value In1 and the second input gradation value In2 arechanged. That is, the pixel positions are fixed in a sequential areaincluding the maximum threshold of the threshold matrix and a sequentialarea including the minimum threshold. Although the threshold areas (In4,In5) in which the overlapping dots are printed are expanded as theoverlapping gradation area width In3 is increased, the separate printingin a threshold area including the maximum threshold of the thresholdmatrix and a threshold area including the minimum threshold is stillperformed. That is, in the case of performing the quantizationprocessing of this embodiment, even if the first input gradation valueIn1 and the second input gradation value In2 are changed in variousways, the pixel positions in which the overlapping dots are printed arestable, and it is possible to print an image with high dispersibilityand no conspicuous granularity.

The above-described difference between the cases of the “quantizationprocessing of Expression (2)” and the quantization processing of thisembodiment may be clearly seen in a case of, particularly, printing animage in which the hue and the density are periodically changed.

FIGS. 15A, 15B, 16A, and 16B are diagrams for comparing cases where thegradation values (In1, In2) having equal values are uniformly inputtedto all the pixels included in a predetermined image area and cases wherethe gradation values are inputted to be varied depending on pixelpositions. FIGS. 15A and 15B show cases of the “quantization processingof Expression (2),” and FIGS. 16A and 16B show cases of the quantizationprocessing of this embodiment.

FIGS. 15A and 16A show results of quantization in a case where the inputgradation values In1=67.5% and In2=67.5% are inputted to all the pixelsincluded in a predetermined image area. On the other hand, FIGS. 15B and16B show results of quantization in a case where the input gradationvalues In1 and In2 are periodically varied between 25% to 100% in apredetermined image area. The drawings each show first color printingpixels, second color printing pixels, and overlapping pixels in whichthe overlapping first color and second color are printed.

In FIG. 15A, the first color is printed in a sequential threshold areaincluding the minimum threshold among all the thresholds arrayed in thefirst threshold matrix, and the second color is printed in a sequentialthreshold area including the maximum threshold among all the thresholdsset in the first threshold matrix. Meanwhile, for the overlappingpixels, the overlapping first color and second color are printed in amiddle threshold area of the entire threshold area, or an area includingneither the minimum threshold nor the maximum threshold, as shown inFIG. 10B.

In FIG. 15B, the first color and the second color are printed at auneven density based on the respective input gradation values. For theoverlapping pixels, the overlapping first color and second color areprinted while a threshold area including neither the minimum thresholdnor the maximum threshold is variously shifted within the entirethreshold area as shown in FIG. 14A.

In contrast, the threshold area of overlapping pixels in which theoverlapping colors are printed in the case of employing the quantizationprocessing of this embodiment is stable in both the case where the inputgradation values of the first color and the second color are fixed asshown in FIG. 16A and case where the input gradation values of the firstcolor and the second color are varied as shown in FIG. 16B. That is, asshown in FIG. 14B, the overlapping colors are printed in a sequentialthreshold area including the minimum threshold and a sequentialthreshold area including the maximum threshold, and consequently, dotarrangements in FIG. 16A and FIG. 16B are equal.

FIG. 17 is a diagram for comparing power distributions of spatialfrequencies in distributions of the overlapping dots of the first andsecond color between the cases of FIGS. 15A, 15B, and 16B. In“quantization processing using inverse relationship,” the power in thecase where the gradation values of the first color and the second colorare varied is increased in the entire area of frequency componentsincluding the low frequency components (varied gradation values) morethan the case where the gradation values of the first color and thesecond color are fixed (fixed gradation values). In contrast, with thequantization processing of this embodiment (the present invention), thepower is not increased even if the gradation values of the first colorand the second color are varied, and the power distributions are almostequal to the case of (fixed gradation values).

In an actual image such as a photograph that is the target of printingby the printing apparatus, the input gradation data is varied within animage area in most cases. If the “quantization processing using inverserelationship” is employed in this case, the threshold area in which theoverlapping dots are printed is varied as shown in FIGS. 14A, 15A, and15B. This increases the power in the entire area of the spatialfrequencies and the granularity may be conspicuous. In contrast, if thequantization processing of this embodiment is employed, the thresholdarea in which the overlapping dots are printed is stable as shown inFIGS. 14B, 16A, and 16B. This suppresses the power in the entire area ofthe spatial frequencies and an image with no conspicuous granularity canbe obtained.

According to the above-described embodiment, for gradation data of thefirst color, quantization is performed according to the first thresholdmatrix having excellent dispersibility on a gradation area in which nooverlapping dots appear. For gradation data of the first color,quantization is performed according to the second threshold matrixhaving the inverse relationship with the first threshold matrix on agradation area in which no overlapping dots appear. For a gradation areain which overlapping dots of the first and second color appear,quantization is performed on a part of the gradation area according tothe first threshold matrix, and quantization is performed on the restpart according to the second threshold matrix. According to thisembodiment, it is possible to stably print the overlapping dots of thefirst and second color in a sequential threshold area including theminimum value and a sequential threshold area including the maximumvalue among the gradation area of the first threshold matrix regardlessof gradation data of the first color and the second color. Consequently,it is possible to obtain high dispersibility in the entire imageincluding overlapping dots regardless of gradation data and to print asmooth image with suppressed granularity.

Second Embodiment

The inkjet printing system shown in FIGS. 1 and 2 is also used in thisembodiment.

FIG. 18 is a flowchart for describing image processing executed by theimage processing apparatus main control unit 108 of this embodiment. Theflowchart of FIG. 18 is different from the flowchart of the firstembodiment shown in FIG. 3 in that index expansion processing S204 isadded. Hereinafter, the steps different from those of the firstembodiment are described.

In step S202, the image processing apparatus main control unit 108 ofthis embodiment separates the converted RGB data into pieces of 16-bitgradation data (density data) of C (cyan), M (magenta), and Y (yellow),which are ink colors of the printing apparatus. In this stage, threechannels (three colors) of 16-bit gray images are generated.

In step S203, the image processing apparatus main control unit 108performs predetermined N-valued quantization processing on the pieces of16-bit gradation data corresponding to the ink colors and generatesquantized data of N gradation. In this embodiment, the data is quantizedto three values, and pieces of 2-bit data of level 0 to level 2 (Lv0,Lv1, and Lv2) are generated based on the 16-bit gradation data.

In the following step S204, the image processing apparatus main controlunit 108 performs the index expansion processing. Specifically, a dotarrangement pattern is selected in association with the level obtainedin step S203 out of multiple dot arrangement patterns in which thenumber and positions of dots printed in the pixels are set in advance,and the selected dot arrangement pattern is generated as dot data. Inthis process, the dot arrangement patterns may be formed such that thenumber of dots printed in an area corresponding to pixels is differentdepending on the level value or may be formed such that the size of thedots is different depending on the level value.

In step S205, the image processing apparatus main control unit 108outputs the dot data generated in S204. With that, the processing ends.

FIG. 19 is a block diagram for describing details of the quantizationprocessing executed in step S203 in FIG. 18 of this embodiment. FIG. 19is different from FIG. 4 described in the first embodiment in that anormalization processing unit 303 is added. Hereinafter, processesrelated to the normalization processing unit 303 are mainly described.

In this embodiment, the gradation data obtainment unit 301 obtains the16-bit gradation data indicating density of each pixel, and the noiseaddition processing unit 302 adds predetermined noise to the 16-bitgradation data.

The normalization processing unit 303 associates the gradation value ofeach pixel represented by 16 bits with the number of gradations (numberof levels) that allows for the index expansion in step S204 andthereafter normalizes a range of each level to 12 bits. A specificdescription is given below. In a case where the index expansionprocessing in step S204 is processing corresponding to N gradations (Nis an integer of 3 or greater) of Lv0 to Lv(N−1), the normalizationprocessing unit 303 divides 65536 gradations represented by 16 bits into(N−1) ranges. Additionally, the normalization processing unit 303normalizes each range to 12 bits (4096 gradations). This makes itpossible to obtain 12-bit data associated with any one of Lv0 to Lv(N−1)for each pixel. In a case where the input value is included in an M-thrange in the order of low to high gradations out of the (N−1) ranges,the input value is quantized by the following quantization processing toeither of a level value (M−1) (Lv(M−1)) or a level value M(LvM).

In this embodiment, since the index expansion processing corresponds tothe three values of Lv0, Lv1, and Lv2, the normalization processing unit303 equally divides 65536 gradations represented by 16 bits into two.Then, the normalization processing unit 303 normalizes gradation values0 to 32767 and gradation values 32768 to 65535 to 12 bits (0 to 4095gradations). Pixels of input gradation values 0 to 32767 as a firstrange are quantized to Lv0 or Lv1 by the following quantizationprocessing, and pixels of input gradation values 32768 to 65535 as asecond range are quantized to Lv1 or Lv2 by the following quantizationprocessing.

The above-described processes by the sequential units from the gradationdata obtainment unit 301 to the normalization processing unit 303 areparallel processed on the pieces of gradation data of different colors,and pieces of 12-bit data of cyan, magenta, and yellow are inputted tothe dither processing unit 311.

FIG. 20 is a schematic diagram for describing the 16-bit input valueinputted to the normalization processing unit 303 and a concept of theprocesses performed by the normalization processing unit 303 and thedither processing unit 311. In the case of normalizing 16 bits (0 to65535) to the three values, the first range is from 0 to 32767 and thesecond range is from 32768 to 65535. The input gradation value Inincluded in the first range is quantized to either of Lv0 and Lv1. Theinput gradation value In included in the second range is quantized toeither of Lv1 and Lv2. In the following description, in each range, asmaller level value is called a quantization reference value BaseLv, andan amount of the input gradation value In that exceeds the quantizationreference value BaseLv is called an intra-range gradation value In′.

In FIG. 20, the size of each range is 100%, and a range that may beoccupied by the input gradation value In is 0 to 200%. There is shown acase of inputting In=125%. In this example, the input gradation value Inis included in the second range, and the quantization reference valueBaseLv corresponds to 100%. The intra-range gradation value In′ isIn′=In−100%=25%, and the input gradation value In is quantized to eitherof Lv1 and Lv2. The intra-range gradation value In′ herein correspondsto the data that is normalized by the normalization processing unit 303.

In this embodiment, the quantization levels of the first color and thesecond color are determined in different ways depending on the magnituderelationship between a quantization reference value BaseLv1 of the firstcolor and a quantization reference value BaseLv2 of the second color.Specifically, two comparative values Comp1A and Comp1B to be used forthe quantization of the first color are set based on the magnituderelationship between the quantization reference value BaseLv1 of thefirst color and the quantization reference value BaseLv2 of the secondcolor. Likewise, two comparative values Comp2A and Comp2B to be used forthe quantization of the second color are set based on the magnituderelationship between the quantization reference value BaseLv1 of thefirst color and the quantization reference value BaseLv2 of the secondcolor.

FIGS. 21A to 21C are diagrams for describing setting examples of thecomparative values Comp1A, Comp1B, Comp2A, and Comp2B. FIG. 21A shows acase of BaseLv1=BaseLv2, FIG. 21B shows a case of BaseLv1<BaseLv2, andFIG. 21C shows a case of BaseLv1>BaseLv2. Descriptions are sequentiallygiven below.

FIG. 21A shows a case where the input gradation value of the first coloris In1=125% and the input gradation value of the second color isIn2=150%. In this case, the quantization reference value and theintra-range gradation value of each color are BaseLv1=BaseLv2=1,In′1=25%, and In′2=50%.

In this embodiment, Comp1A in any condition is Comp1A=In′1.Additionally, Comp2B in any condition is Comp2B=In′2.

On the other hand, Comp1B is the input gradation value In2 (In′2) of thesecond color in a range including the input gradation value In1 of thefirst color. That is, if BaseLv1 and BaseLv2 have equal values like FIG.21A, Comp1B=In′2. Additionally, Comp2A is the input gradation value In1(In′1) of the first color in a range including the input gradation valueIn2 of the second color. That is, if BaseLv1 and BaseLv2 have equalvalues like FIG. 21A, Comp2A=In′1.

FIG. 21B shows a case where the input gradation value of the first coloris In1=25% and the input gradation value of the second color isIn2=150%. In this case, the quantization reference value and theintra-range gradation value of each color are BaseLv1=0, BaseLv2=1,In′1=25%, and In′2=50%. The intra-range gradation value In′2 of thesecond color in the first range including the input gradation value In1of the first color is 100%, and the intra-range gradation value In′1 ofthe first color in the second range including the input gradation valueIn2 of the second color is 0%. Thus, the four comparative values areComp1A=25%, Comp1B=100%, Comp2A=0%, and Comp2B=In′2.

FIG. 21C shows a case where the input gradation value of the first coloris In1=125% and the input gradation value of the second color isIn2=50%. In this case, the quantization reference value and theintra-range gradation value of each color are BaseLv1=1, BaseLv2=0,In′1=25%, and In′2=50%. The intra-range gradation value In′2 of thesecond color in the second range including the input gradation value In1of the first color is 0%, and the intra-range gradation value In′1 ofthe first color in the first range including the input gradation valueIn2 of the second color is 100%. Thus, the four comparative values areComp1A=25%, Comp1B=0%, Comp2A=100%, and Comp2B=In′2.

FIG. 22 shows the above-described correspondence between magnituderelationship between the quantization reference values and the fourcomparative values Comp1A, Comp1B, Comp2A, and Comp2B.

FIG. 23 is a flowchart for describing the processing for thequantization of the first color executed by the dither processing unit311 of this embodiment. Once the processing starts, first, in S30, thedither processing unit 311 obtains the threshold Dth1 of the firstthreshold matrix, the threshold Dth2 of the second threshold matrix, andthe two comparative values Comp1A and Comp1B. Dth1 and Dth2 may bethresholds for the processing-target pixel respectively read from thethresholds arrayed in the first threshold matrix and the secondthreshold matrix. The comparative values Comp1A and Comp1B may be setaccording to the table shown in FIG. 22 using the input gradation valuesIn1 and In2 inputted by the normalization processing unit 303. Notethat, like the first embodiment, the first threshold matrix and thesecond threshold matrix have the inverse relationship.

In S31, the dither processing unit 311 determines whether the followinginequality is true:

Comp1A+Comp1B>100%.

If it is determined that the above inequality is not true, the processproceeds to S32, and if it is determined that the above inequality istrue, the process proceeds to S35.

The processes in S32 and following steps are for performing thequantization on the first color with the first threshold matrix. Thatis, the intra-range gradation value In′1 of the first color is comparedwith the threshold Dth1 of the first threshold matrix, and if In′1>Dth1,the process proceeds to S33 and the quantization level of the firstcolor is set to BaseLv1+1. If In′1<Dth1, the process proceeds to S34 andthe quantization level of the first color is set to BaseLv1.

On the other hand, the processes in S35 and following steps are forperforming the characteristic quantization of the present invention onthe first color. In S35, the dither processing unit 311 obtains anoverlapping correction gradation area width In′3. The overlappingcorrection gradation area width In′3 corresponds to a width of thethreshold area in which both the first color and second color areprinted and can be expressed by the following expression:

In′3=Comp1A+Comp1B−100%.

Additionally, in S36, the dither processing unit 311 equally divides theoverlapping correction gradation area width In′3 into two to calculate afirst overlapping area width In′4:

In′4=In′3/2.

If the thus-obtained value includes a fraction, it is cut off to obtainIn′4.

In S37, the dither processing unit 311 determines whether the firstoverlapping area width In′4 satisfies In′4>Dth2 (Condition 4) orComp1A−In′4>Dth1 (Condition 5). If the first overlapping area width In′4satisfies either of Condition 4 and Condition 5, the process proceeds toS38 and the quantization level (Out1) of the first color in theprocessing-target pixel is set to BaseLv1+1. The pixel satisfyingCondition 4 is a pixel in which both the first and second colors areprinted at the same quantization level according to the second thresholdmatrix, and the pixel satisfying Condition 5 is a pixel in which thequantization level of the first color has a higher level value than thequantization level of the second color for the printing. In S37, if thefirst overlapping area width In′4 satisfies neither Condition 4 norCondition 5, the process proceeds to S39 and the quantization level(Out1) of the first color in the processing-target pixel is set toBaseLv1.

FIG. 24 is a flowchart for describing the processing for thequantization of the second color executed by the dither processing unit311 of this embodiment. Once the processing starts, first, in S40, thedither processing unit 311 obtains the threshold Dth1 of the firstthreshold matrix, the threshold Dth2 of the second threshold matrix, andthe two comparative values Comp2A and Comp2B. Dth1 and Dth2 may bethresholds for the processing-target pixel respectively read from thethresholds arrayed in the first threshold matrix and the secondthreshold matrix. The comparative values Comp2A and Comp2B may be setaccording to the table shown in FIG. 22 using the input gradation valuesIn1 and In2 inputted by the normalization processing unit 303. Notethat, like the first embodiment, the first threshold matrix and thesecond threshold matrix have the inverse relationship.

In S41, the dither processing unit 311 determines whether the followinginequality is true:

Comp2A+Comp2B>100%.

If it is determined that the above inequality is not true, the processproceeds to S42, and if it is determined that the above inequality istrue, the process proceeds to S45.

The processes in S42 and following steps are for performing thequantization on the second color with the second threshold matrix. Thatis, the intra-range gradation value In′2 of the second color is comparedwith the threshold Dth2 of the second threshold matrix, and ifIn′2>Dth2, the process proceeds to S43 and the quantization level of thesecond color is set to BaseLv2+1. If In′2<Dth2, the process proceeds toS44 and the quantization level of the second color is set to BaseLv2.

On the other hand, the processes in S45 and following steps are forperforming the characteristic quantization of the present invention onthe second color. In S45, the dither processing unit 311 obtains anoverlapping correction gradation area width In′6. The overlappingcorrection gradation area width In′6 corresponds to a width of thethreshold area in which both the first color and second color areprinted and can be expressed by the following expression:

In′6=Comp2A+Comp2B−100%.

In S46, the dither processing unit 311 equally divides the overlappingcorrection gradation area width In′6 into two to calculate a thirdoverlapping area width In′7:

In′7=In′3/2.

If the thus-obtained value includes a fraction, it is cut off to obtainIn′7.

Additionally, in S47, the dither processing unit 311 subtracts the thirdoverlapping area width In′7 from the overlapping correction gradationarea width In′6 to calculate a fourth overlapping area width In′8:

In′8=In′6−In′7

The fraction that is cut off in the division in S46 is included in thefourth overlapping area width In′8.

In following S48, the dither processing unit 311 determines whether thefourth overlapping area width In′8 satisfies In′8>Dth1 (Condition 6) orComp2B−In′8>Dth2 (Condition 7). If the fourth overlapping area widthIn′8 satisfies either of Condition 6 and Condition 7, the processproceeds to S49 and the quantization level (Out2) of the second color inthe processing-target pixel is set to BaseLv2+1. The pixel satisfyingCondition 6 is a pixel in which both the first and second colors areprinted at the same quantization level according to the first thresholdmatrix, and the pixel satisfying Condition 7 is a pixel in which thequantization level of the second color has a higher level value than thequantization level of the first color for the printing. In S48, if thefourth overlapping area width In′8 satisfies neither Condition 6 norCondition 7, the process proceeds to S50 and the quantization level(Out2) of the second color in the processing-target pixel is set toBaseLv2.

Hereinafter, with reference to FIGS. 23 and 24 again, descriptions aregiven for each case of FIGS. 21A to 21C assuming that pieces ofgradation data having the same values are uniformly inputted to arelatively large image area. In the case of BaseLv1=BaseLv2, the pixelin which the first color is printed and the pixel in which the secondcolor is printed in the same range are arranged like the firstembodiment. For example, in the case of BaseLv1=BaseLv2=1 as shown inFIG. 21A, while the first and second colors are uniformly printed at thequantization level Lv1 in the entire pixel area, the first and secondcolors of Lv2 are exclusively printed in a relatively low gradation, andprinting of almost half of overlapping pixels is made according to thefirst threshold matrix and printing of the other half of the overlappingpixels is made according to the second threshold matrix in a relativelyhigh gradation.

In a case of BaseLv1=BaseLv2=0, the first and second colors of Lv1 areexclusively printed in a blank area in a relatively low gradation, andprinting of almost half of overlapping pixels is made according to thefirst threshold matrix and printing of the other half of the overlappingpixels is made according to the second threshold matrix in a relativelyhigh gradation.

In the case of BaseLv1<BaseLv2 shown in FIG. 21B, while the second coloris uniformly printed at the quantization level Lv1 or Lv2 in the entirepixel area, the first color of Lv1 is printed to overlap the secondcolor, and overlapping pixels are thus formed. In this case, for thefirst color, since Comp1B is set to 100% according to FIG. 22, S31 inFIG. 23 is inevitably Yes and the process proceeds to S35. Then, in thefollowing processes, printing of almost half of the overlapping pixelsis made according to the second threshold matrix (Condition 4) with highdispersibility, and printing of the other half of the overlapping pixelsis made according to the first threshold matrix (Condition 5) with highdispersibility. On the other hand, for the second color of Lv2, sinceComp2A is set to 0% according to FIG. 22, S41 in FIG. 24 is inevitablyNo and the process proceeds to S42 to make printing according to thesecond threshold matrix.

In the case of BaseLv1>BaseLv2 shown in FIG. 21C, while the first coloris uniformly printed at the quantization level Lv1 or Lv2 in the entirepixel area, the second color of Lv1 is printed to overlap the firstcolor, and overlapping pixels are thus formed. In this case, for thefirst color, since Comp1B is set to 0% according to FIG. 22, S31 in FIG.23 is inevitably No and the process proceeds to S32 to make printingaccording to the first threshold matrix. On the other hand, for thesecond color of Lv2, since Comp2A is set to 100% according to FIG. 22,S41 in FIG. 24 is inevitably Yes and the process proceeds to S45. Then,in the following processes, printing of almost half of the overlappingpixels is made according to the first threshold matrix (Condition 6)with high dispersibility, and printing of the other half of theoverlapping pixels is made according to the second threshold matrix(Condition 7) with high dispersibility.

According to the above-described embodiment, in any cases of FIGS. 21Ato 21C, it is possible to print almost half of the overlapping pixels ofthe first and second colors according to the first threshold matrix andto print the other half of the overlapping pixels of the first andsecond colors according to the second threshold matrix.

That is, according to this embodiment, in the printing pixels havingmultiple quantization levels, the quantization processing according to asequential gradation area including the minimum threshold and asequential gradation area including the maximum threshold can beperformed on a pixel having higher quantization level (i.e., pixelhaving higher dot power). Consequently, this makes it possible to obtaina smooth image with no conspicuous granularity in a wide range ofgradation areas expressed by multiple quantization levels.

Third Embodiment

The similar inkjet printing system as that of the second embodiment isalso used in this embodiment, and image processing is performed inaccordance with the flowcharts in FIGS. 18, 23 and 24 like the secondembodiment. The difference between this embodiment and the secondembodiment is a setting method of the four comparative values Comp1A,Comp1B, Comp2A and Comp2B.

FIG. 25 shows correspondence between the magnitude relationship betweenthe quantization reference values and the four comparative valuesComp1A, Comp1B, Comp2A and Comp2B of this embodiment. In thisembodiment, Comp1A=In′1, Comp1B=In′2, Comp2A=In′1, and Comp2B=In′2 inthe case of BaseLv1=BaseLv2, and Comp1A=In′1, Comp1B=0%, Comp2A=0%, andComp2B=In′2 in the case of BaseLv1≠BaseLv2.

According to this embodiment, in the case of BaseLv1≠BaseLv2, theprocess always proceeds to S32 in FIG. 23 and the first color isquantized with the first threshold matrix, while the process alwaysproceeds to S42 in FIG. 24 and the second color is quantized with thesecond threshold matrix. Thus, in the case of BaseLv1<BaseLv2, all ofthe overlapping pixels are arranged using the first threshold matrix,and in the case of BaseLv1>BaseLv2, all of the overlapping pixels arearranged using the second threshold matrix. That is, in the case ofBaseLv1≠BaseLv2 in this embodiment, overlapping pixels are arrangedaccording to either of the first threshold matrix and the secondthreshold matrix. Even in this case, it is still possible tosufficiently suppress the granularity more than the conventionalconfiguration in which overlapping pixels include neither the minimumthreshold nor the maximum threshold and sequential threshold areas areunstable depending on the gradation values of the first color and thesecond color. Additionally, according to this embodiment, it is possibleto reduce the number of computations in S35 and following steps in FIG.23 and the number of computations in S45 and following steps in FIG. 24,and the quantization processing can be performed faster than the secondembodiment.

Fourth Embodiment

In the above-described embodiments, a gradation value of the secondcolor is corrected (shifted) with a predetermined method and comparedwith a fixed threshold in order to arrange overlapping pixels of thefirst and second color according to the order of the first thresholdmatrix. In contrast, in this embodiment, a threshold is corrected(shifted) in order to obtain the same effects as the above-describedembodiments.

FIG. 26 is a block diagram for describing a control configuration of adither processing unit 311 employed for this embodiment. In thisembodiment, configurations of the dither pattern 310 and the thresholdobtainment unit 305 are the same as those of the above-describedembodiments. That is, the dither pattern 310 stores the first thresholdmatrix having blue noise characteristics and the second threshold matrixhaving the inverse relationship with the first threshold matrix. Thethreshold obtainment unit 305 obtains the thresholds Dth1 and Dth2corresponding to the processing-target pixel respectively from thethresholds arrayed in the first threshold matrix and the secondthreshold matrix and transmits the thresholds Dth1 and Dth2 to athreshold offset amount addition unit 309.

The inter-color processing unit 304 of this embodiment includes athreshold offset amount calculation unit 308 and the threshold offsetamount addition unit 309. The threshold offset amount calculation unit308 calculates a threshold offset amount ThOft1 or ThOft2 as acorrection amount for correcting a threshold based on the inputgradation values In1 and In2 of the first and second colors andtransmits the threshold offset amount ThOft1 or ThOft2 to the thresholdoffset amount addition unit 309. The threshold offset amount additionunit 309 adds the threshold offset amount ThOft1 or ThOft2 received fromthe threshold offset calculation unit 308 to the threshold received fromthe threshold obtainment unit 305 to obtain a corrected threshold Dth′1for the first color or a corrected threshold Dth′2 for the second color,and transmits the obtained value to the quantization processing unit306. The quantization processing unit 306 compares the input gradationvalue In1 of the first color or the input gradation value In2 of thesecond color with the corrected threshold Dth′1 for the first color orthe corrected threshold Dth′2 for the second color to determine thequantization level, and outputs the determined quantization level.

FIG. 27 is a flowchart for describing the processing for thequantization of the input gradation value In1 of the first color and thegradation value In2 of the second color in the processing-target pixelexecuted by the dither processing unit 311 of this embodiment. Once theprocessing starts, first, in S400, the dither processing unit 311obtains the threshold Dth1 of the first threshold matrix, the thresholdDth2 of the second threshold matrix, and the quantization referencevalues BaseLv1 and BaseLv2 of the first and second colors. Dth1 and Dth2may be thresholds for the processing-target pixel respectively read fromthe thresholds arrayed in the first threshold matrix and the secondthreshold matrix. The quantization reference values BaseLv1 and BaseLv2are obtained based on the input gradation values In1 and In2 of thefirst and second colors like the second embodiment.

S401 to S405 are steps for calculating an input addition value InSumaccording to the following expressions based on the magnituderelationship between BaseLv1 and BaseLv2:

in the case of BaseLv1=BaseLv2 (S402),

InSum1=In′1+In′2, and

InSum2=In′1+In′2;

in the case of BaseLv1<BaseLv2 (S404),

InSum1=In′1+Dthmax, and

InSum2=0+In′2; and

in the case of BaseLv1>BaseLv2 (S405),

InSum1=In′1+0,

InSum2=Dthmax+In′2,

where Dthmax is the maximum value of the thresholds arrayed in thethreshold matrix, and Dthmax=4095 in this embodiment.

In the following processes, S406 to S409 are steps for obtaining thecorrected threshold Dth′1 for the first color (hereinafter, firstcorrected threshold), and S410 to S413 are steps for obtaining thecorrected threshold Dth′2 for the second color (hereinafter, secondcorrected threshold).

In S406, if InSum1>Dthmax is true, the dither processing unit 311 allowsthe process to proceed to S407 and calculates the first threshold offsetamount ThOft1 according to the following expression:

ThOft1=(InSum1−Dthmax)/2.

If the thus-obtained value includes a fraction, it is cut off to obtainThOft1.

On the other hand, if InSum1>Dthmax is not true, the process proceeds toS408 and the first threshold offset amount ThOft1 is set to 0.

In S409, the dither processing unit 311 adds the first threshold offsetamount ThOft1 to the threshold Dth1 of the first color obtained in S400and obtains the first corrected threshold Dth′1:

Dth′1=Dth1+ThOft1.

In S410, if InSum2>Dthmax is true, the dither processing unit 311 allowsthe process to proceed to S411 and calculates the second thresholdoffset amount ThOft2 according to the following expression:

ThOft2=(InSum2−Dthmax+1)/2.

If the thus-obtained value includes a fraction, it is cut off to obtainThOft2.

On the other hand, if InSum2>Dthmax is not true, the process proceeds toS412 and the second threshold offset amount ThOft2 is set to 0.

In S413, the dither processing unit 311 adds the second threshold offsetamount ThOft2 to the threshold Dth2 of the second color obtained in S400and obtains the second corrected threshold Dth′2:

Dth′2=Dth2+ThOft2.

In S414, the dither processing unit 311 performs the quantizationprocessing on the first color and the second color. That is, thequantization level of the first color is set by comparing In′1 withDth′1, and the quantization level of the second color is set bycomparing In′2 with Dth′2. In this process, for the first color, ifDth′1 calculated in S409 is equal to or smaller than Dthmax, thequantization level is directly set by comparing In′1 with Dth′1. On theother hand, if Dth′1 is greater than Dthmax, a value obtained bysubtracting Dthmax+1 from Dth′1 is set as new Dth′1, and thequantization level is set by comparing this new Dth′1 with In′1.

For the second color, if Dth′2 calculated in S413 is equal to or smallerthan Dthmax, the quantization level is directly set by comparing In′2with Dth′2. On the other hand, if Dth′2 is greater than Dthmax, a valueobtained by subtracting Dthmax+1 from Dth′2 is set as new Dth′2, and thequantization level is set by comparing this new Dth′2 with In′2. Withthat, the processing ends.

According to the above-described embodiment, it is possible to obtainthe similar effect as that of the second embodiment by correcting(shifting) a threshold instead of an input gradation value. Although thedescribed method is a method enabling the quantization processingequivalent to that of the second embodiment, such a “method ofcorrecting a threshold” can be also applied to the first and thirdembodiments, and the similar effects as those of the embodiments can beobtained. Additionally, the circuit for implementing this embodimenthaving the control configuration shown in FIG. 26 can be smaller thanthe circuits for correcting an input gradation value of the first tothird embodiments, and thus the cost of the apparatus can be reduced.

Other Embodiments

Although the case where the first color is cyan and the second color ismagenta is described as an example, it is needless to say that thepresent invention is not limited to this combination. In thequantization processing of the above-described embodiments, the casewhere the first color and the second color have equal dot powers isdescribed. It is preferable to set a color having lower lightness andhigher dot power to the first color on which the quantization processingaccording to the threshold matrix can be preferentially performed, andif the dot power of magenta is higher than that of cyan, the first colormay be magenta and the second color may be cyan. Otherwise, it ispossible to set cyan or magenta to either one of the first and secondcolors and replace the other one with a different color such as yellow,and this may also be applied to a case of using an ink of different huesuch as red, blue, green, black, and so on.

Although the case of using the inkjet printing apparatus is described asan example, the present invention is not limited thereto. A differenttype of printing apparatus can exert the effects of the presentinvention as long as it is a color printing apparatus capable of makingpseudo-gradation expression by printing dots.

The present invention may be implemented by processing that is executedsuch that a program implementing one or more functions of theabove-described embodiments is supplied to a system or an apparatusthrough a network or a storage medium and one or more processors in acomputer of the system or the apparatus read the program. Otherwise, thepresent invention may be implemented by a circuit implementing one ormore functions (e.g., ASIC).

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2018-145330 filed Aug. 1, 2019, which is hereby incorporated byreference wherein in its entirety.

What is claimed is:
 1. An image processing apparatus, comprising: agradation data obtainment unit configured to obtain first gradation datacorresponding to a gradation value of a first color and second gradationdata corresponding to a gradation value of a second color for aprocessing-target pixel; a threshold obtainment unit configured toobtain a first threshold for the processing-target pixel from a firstthreshold matrix including a plurality of arrayed thresholds for pixelsand obtain a second threshold for the processing-target pixel from asecond threshold matrix in which the thresholds for the pixels arearrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generation unit configured to generate firstquantization data and second quantization data that have a smallernumber of gradations than the number of gradations of the firstgradation data and the second gradation data based on the firstthreshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing apparatus performing imageprocessing to print a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein the generationunit, in a case where a sum of the first gradation data and the secondgradation data is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, generates the firstquantization data based on a result of comparing the first gradationdata with the first threshold and generates the second quantization databased on a result of comparing the second gradation data with the secondthreshold, and in a case where the sum is greater than the maximumvalue, generates first overlapping gradation data and second overlappinggradation data by dividing a value that is obtained by subtracting themaximum value from the sum into two, generates the first quantizationdata based on a result of comparing the first overlapping gradation datawith the second threshold or a result of comparing a difference betweenthe first gradation data and the first overlapping gradation data withthe first threshold, and generates the second quantization data based ona result of comparing the second overlapping gradation data with thefirst threshold or a result of comparing a difference between the secondgradation data and the second overlapping gradation data with the secondthreshold.
 2. An image processing apparatus, comprising: a gradationdata obtainment unit configured to obtain first gradation datacorresponding to a gradation value of a first color and second gradationdata corresponding to a gradation value of a second color for aprocessing-target pixel; a threshold obtainment unit configured toobtain a first threshold for the processing-target pixel from a firstthreshold matrix including a plurality of arrayed thresholds for pixelsand obtain a second threshold for the processing-target pixel from asecond threshold matrix in which the thresholds for the pixels arearrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generation unit configured to generate firstquantization data and second quantization data that have a smallernumber of gradations than the number of gradations of the firstgradation data and the second gradation data based on the firstthreshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing apparatus performing imageprocessing to print a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein the generationunit, in a case where a sum of the first gradation data and the secondgradation data is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, generates the firstquantization data based on a result of comparing the first gradationdata with the first threshold and generates the second quantization databased on a result of comparing the second gradation data with the secondthreshold, and in a case where the sum is greater than the maximumvalue, generates a first correction amount and a second correctionamount by dividing a value that is obtained by subtracting the maximumvalue from the sum into two, generates the first quantization data bycomparing the first gradation data with a value obtained by adding thefirst correction amount to the first threshold and generates the secondquantization data by comparing the second gradation data with a valueobtained by adding the second correction amount to the second threshold.3. The image processing apparatus according to claim 1, wherein thefirst quantization data is binary data that determines whether or not toprint the color material of the first color, and the second quantizationdata is binary data that determines whether or not to print the colormaterial of the second color.
 4. An image processing apparatus,comprising: a gradation data obtainment unit configured to obtain firstgradation data corresponding to a gradation value of a first color andsecond gradation data corresponding to a gradation value of a secondcolor for a processing-target pixel; a threshold obtainment unitconfigured to obtain a first threshold for the processing-target pixelfrom a first threshold matrix including a plurality of arrayedthresholds for pixels and obtain a second threshold for theprocessing-target pixel from a second threshold matrix in which thethresholds for the pixels are arrayed at such pixel positions that orderof the pixel positions is inverse to order of pixel positions in thefirst threshold matrix in a case where the pixel positions are arrangedin ascending order of the thresholds; and a generation unit configuredto generate first quantization data and second quantization data thathave N gradations (N is an integer equal to or greater than 3), with Nbeing smaller than the number of gradations of the first gradation dataand the second gradation data, based on the first threshold, the secondthreshold, the first gradation data, and the second gradation data: theimage processing apparatus performing image processing to print a colormaterial of the first color based on the first quantization data and toprint a color material of the second color based on the secondquantization data, wherein the generation unit equally divides agradation domain of the first gradation data and the second gradationdata into (N−1) ranges to calculate a first intra-range gradation valuethat is a gradation value of the first gradation data in a rangeincluding the first gradation data and calculate a second intra-rangegradation value that is a gradation value of the second gradation datain a range including the second gradation data, and in a case where therange including the first gradation data and the range including thesecond gradation data are equal, i) in a case where a sum of the firstintra-range gradation value and the second intra-range gradation valueis equal to or smaller than the maximum value of the thresholds arrayedin the first threshold matrix, generates the first quantization databased on a result of comparing the first intra-range gradation valuewith the first threshold and generates the second quantization databased on a result of comparing the second intra-range gradation valuewith the second threshold, and ii) in a case where the sum is greaterthan the maximum value, generates first overlapping gradation data andsecond overlapping gradation data by dividing a value that is obtainedby subtracting the maximum value from the sum into two, generates thefirst quantization data based on a result of comparing the firstoverlapping gradation data with the second threshold or a result ofcomparing a difference between the first intra-range gradation value andthe first overlapping gradation data with the first threshold, andgenerates the second quantization data based on a result of comparingthe second overlapping gradation data with the first threshold or aresult of comparing a difference between the second intra-rangegradation value and the second overlapping gradation data with thesecond threshold.
 5. The image processing apparatus according to claim4, wherein the generation unit is configured to, in a case where therange including the first gradation data is lower in gradation than therange including the second gradation data, generates the firstquantization data based on the result of comparing the first overlappinggradation data with the second threshold or the result of comparing adifference between the first intra-range gradation value and the firstoverlapping gradation data with the first threshold and generates thesecond quantization data based on the result of comparing the secondintra-range gradation value with the second threshold, and in a casewhere the range including the first gradation data is higher ingradation than the range including the second gradation data, generatesthe first quantization data based on the result of comparing the firstintra-range gradation value with the first threshold and generates thesecond quantization data based on the result of comparing the secondoverlapping gradation data with the first threshold or the result ofcomparing a difference between the second intra-range gradation valueand the second overlapping gradation data with the second threshold. 6.The image processing apparatus according to claim 4, wherein thegeneration unit, in a case where the range including the first gradationdata is different from the range including the second gradation data,generates the first quantization data based on the result of comparingthe first intra-range gradation value with the first threshold andgenerates the second quantization data based on the result of comparingthe second intra-range gradation value with the second threshold.
 7. Animage processing apparatus, comprising: a gradation data obtainment unitconfigured to obtain first gradation data corresponding to a gradationvalue of a first color and second gradation data corresponding to agradation value of a second color for a processing-target pixel; athreshold obtainment unit configured to obtain a first threshold for theprocessing-target pixel from a first threshold matrix including aplurality of arrayed thresholds for pixels and obtain a second thresholdfor the processing-target pixel from a second threshold matrix in whichthe thresholds for the pixels are arrayed at such pixel positions thatorder of the pixel positions is inverse to order of pixel positions inthe first threshold matrix in a case where the pixel positions arearranged in ascending order of the thresholds; and a generation unitconfigured to generate first quantization data and second quantizationdata that have N gradations (N is an integer equal to or greater than3), with N being smaller than the number of gradations of the firstgradation data and the second gradation data, based on the firstthreshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing apparatus performing imageprocessing to print a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein the generation unitequally divides a gradation domain of the first gradation data and thesecond gradation data into (N−1) ranges to calculate a first intra-rangegradation value that is a gradation value of the first gradation data ina range including the first gradation data and calculate a secondintra-range gradation value that is a gradation value of the secondgradation data in a range including the second gradation data, and in acase where the range including the first gradation data and the rangeincluding the second gradation data are equal, i) in a case where a sumof the first intra-range gradation value and the second intra-rangegradation value is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, generates the firstquantization data based on a result of comparing the first intra-rangegradation value with the first threshold and generates the secondquantization data based on a result of comparing the second intra-rangegradation value with the second threshold, and ii) in a case where thesum is greater than the maximum value, generates a first thresholdoffset amount and a second threshold offset amount by dividing a valuethat is obtained by subtracting the maximum value from the sum into two,generates the first quantization data based on a result of comparing thefirst intra-range gradation value with a value obtained by adding thefirst threshold offset amount to the first threshold and generates thesecond quantization data based on a result of comparing the secondintra-range gradation value with a value obtained by adding the secondthreshold offset amount to the second threshold.
 8. The image processingapparatus according to claim 7, wherein the generation unit in a casewhere the range including the first gradation data is lower in gradationthan the range including the second gradation data, generates the firstquantization data based on a result of comparing the first intra-rangegradation value with a sum of a value obtained by dividing the firstintra-range gradation value into two and the first threshold andgenerates the second quantization data based on the result of comparingthe second intra-range gradation value with the second threshold, and ina case where the range including the first gradation data is higher ingradation than the range including the second gradation data, generatesthe first quantization data based on the result of comparing the firstintra-range gradation value with the first threshold and generates thesecond quantization data based on a result of comparing the secondintra-range gradation value with a sum of a value obtained by dividingthe second intra-range gradation value into two and the secondthreshold.
 9. The image processing apparatus according to claim 7,wherein the generation, in a case where the range including the firstgradation data is different in gradation from the range including thesecond gradation data, generates the first quantization data based onthe result of comparing the first intra-range gradation value with thefirst threshold and generates the second quantization data based on theresult of comparing the second intra-range gradation value with thesecond threshold.
 10. The image processing apparatus according to claim4, wherein the generation unit, in a case where the first gradation datais included in an M-th range in ascending order of gradations in the(N−1) ranges, generates a level value (M−1) or a level value M as thefirst quantization data, and in a case where the second gradation datais included in an L-th range in ascending order of gradations in the(N−1) ranges, generates a level value (L−1) or a level value L as thesecond quantization data.
 11. The image processing apparatus accordingto claim 1, wherein a lightness of the first color is lower than alightness of the second color.
 12. The image processing apparatusaccording to claim 1, wherein either one of the first color and thesecond color is cyan and the other is magenta.
 13. The image processingapparatus according to claim 1, wherein the first threshold matrix hasblue noise characteristics.
 14. The image processing apparatus accordingto claim 1, wherein the second threshold for the processing-target pixelis obtained by subtracting the first threshold for the processing-targetpixel from the maximum value of the thresholds arrayed in the firstthreshold matrix.
 15. The image processing apparatus according to claim1, further comprising: a printing unit configured to print the colormaterial of the first color based on the first quantization data and toprint the color material of the second color based on the secondquantization data.
 16. An image processing method, comprising: agradation data obtaining step of obtaining first gradation datacorresponding to a gradation value of a first color and second gradationdata corresponding to a gradation value of a second color for aprocessing-target pixel; a threshold obtaining step of obtaining a firstthreshold for the processing-target pixel from a first threshold matrixincluding a plurality of arrayed thresholds for pixels and obtain asecond threshold for the processing-target pixel from a second thresholdmatrix in which the thresholds for the pixels are arrayed at such pixelpositions that order of the pixel positions is inverse to order of pixelpositions in the first threshold matrix in a case where the pixelpositions are arranged in ascending order of the thresholds; and agenerating step of generating first quantization data and secondquantization data that have a smaller number of gradations than thenumber of gradations of the first gradation data and the secondgradation data based on the first threshold, the second threshold, thefirst gradation data, and the second gradation data: the imageprocessing method performing image processing for printing a colormaterial of the first color based on the first quantization data and toprint a color material of the second color based on the secondquantization data, wherein in the generating step, in a case where a sumof the first gradation data and the second gradation data is equal to orsmaller than the maximum value of the thresholds arrayed in the firstthreshold matrix, the first quantization data is generated based on aresult of comparing the first gradation data with the first thresholdand the second quantization data is generated based on a result ofcomparing the second gradation data with the second threshold, and inthe generating step, in a case where the sum is greater than the maximumvalue, first overlapping gradation data and second overlapping gradationdata are generated by dividing a value that is obtained by subtractingthe maximum value from the sum into two, the first quantization data isgenerated based on a result of comparing the first overlapping gradationdata with the second threshold or a result of comparing a differencebetween the first gradation data and the first overlapping gradationdata with the first threshold, and the second quantization data isgenerated based on a result of comparing the second overlappinggradation data with the first threshold or a result of comparing adifference between the second gradation data and the second overlappinggradation data with the second threshold.
 17. An image processingmethod, comprising: a gradation data obtaining step of obtaining firstgradation data corresponding to a gradation value of a first color andsecond gradation data corresponding to a gradation value of a secondcolor for a processing-target pixel; a threshold obtaining step ofobtaining a first threshold for the processing-target pixel from a firstthreshold matrix including a plurality of arrayed thresholds for pixelsand obtain a second threshold for the processing-target pixel from asecond threshold matrix in which the thresholds for the pixels arearrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have a smaller number of gradationsthan the number of gradations of the first gradation data and the secondgradation data based on the first threshold, the second threshold, thefirst gradation data, and the second gradation data: the imageprocessing method further comprising an image processing step ofprinting a color material of the first color based on the firstquantization data and to print a color material of the second colorbased on the second quantization data, wherein in the generating step,in a case where a sum of the first gradation data and the secondgradation data is equal to or smaller than the maximum value of thethresholds arrayed in the first threshold matrix, the first quantizationdata is generated based on a result of comparing the first gradationdata with the first threshold and the second quantization data isgenerated based on a result of comparing the second gradation data withthe second threshold, and in a case where the sum is greater than themaximum value, a first correction amount and a second correction amountare generated by dividing a value that is obtained by subtracting themaximum value from the sum into two, the first quantization data isgenerated by comparing the first gradation data with a value obtained byadding the first correction amount to the first threshold, and thesecond quantization data is generated by comparing the second gradationdata with a value obtained by adding the second correction amount to thesecond threshold.
 18. An image processing method, comprising: agradation data obtaining step of obtaining first gradation datacorresponding to a gradation value of a first color and second gradationdata corresponding to a gradation value of a second color for aprocessing-target pixel; a threshold obtaining step of obtaining a firstthreshold for the processing-target pixel from a first threshold matrixincluding a plurality of arrayed thresholds for pixels and obtain asecond threshold for the processing-target pixel from a second thresholdmatrix in which the thresholds for the pixels are arrayed at such pixelpositions that order of the pixel positions is inverse to order of pixelpositions in the first threshold matrix in a case where the pixelpositions are arranged in ascending order of the thresholds; and agenerating step of generating first quantization data and secondquantization data that have N gradations (N is an integer equal to orgreater than 3), with N being smaller than the number of gradations ofthe first gradation data and the second gradation data, based on thefirst threshold, the second threshold, the first gradation data, and thesecond gradation data: the image processing method performing imageprocessing for printing a color material of the first color based on thefirst quantization data and to print a color material of the secondcolor based on the second quantization data, wherein in the generatingstep, a gradation domain of the first gradation data and the secondgradation data is equally divided into (N−1) ranges to calculate a firstintra-range gradation value that is a gradation value of the firstgradation data in a range including the first gradation data andcalculate a second intra-range gradation value that is a gradation valueof the second gradation data in a range including the second gradationdata, and in a case where the range including the first gradation dataand the range including the second gradation data are equal, i) in acase where a sum of the first intra-range gradation value and the secondintra-range gradation value is equal to or smaller than the maximumvalue of the thresholds arrayed in the first threshold matrix, the firstquantization data is generated based on a result of comparing the firstintra-range gradation value with the first threshold and the secondquantization data is generated based on a result of comparing the secondintra-range gradation value with the second threshold, and ii) in a casewhere the sum is greater than the maximum value, first overlappinggradation data and second overlapping gradation data are generated bydividing a value that is obtained by subtracting the maximum value fromthe sum into two, the first quantization data is generated based on aresult of comparing the first overlapping gradation data with the secondthreshold or a result of comparing a difference between the firstintra-range gradation value and the first overlapping gradation datawith the first threshold, and the second quantization data is generatedbased on a result of comparing the second overlapping gradation datawith the first threshold or a result of comparing a difference betweenthe second intra-range gradation value and the second overlappinggradation data with the second threshold.
 19. An image processingmethod, comprising: a gradation data obtaining step of obtaining firstgradation data corresponding to a gradation value of a first color andsecond gradation data corresponding to a gradation value of a secondcolor for a processing-target pixel; a threshold obtaining step ofobtaining a first threshold for the processing-target pixel from a firstthreshold matrix including a plurality of arrayed thresholds for pixelsand obtain a second threshold for the processing-target pixel from asecond threshold matrix in which the thresholds for the pixels arearrayed at such pixel positions that order of the pixel positions isinverse to order of pixel positions in the first threshold matrix in acase where the pixel positions are arranged in ascending order of thethresholds; and a generating step of generating first quantization dataand second quantization data that have N gradations (N is an integerequal to or greater than 3), with N being smaller than the number ofgradations of the first gradation data and the second gradation data,based on the first threshold, the second threshold, the first gradationdata, and the second gradation data: the image processing methodperforming image processing for printing a color material of the firstcolor based on the first quantization data and to print a color materialof the second color based on the second quantization data, wherein inthe generating step, a gradation domain of the first gradation data andthe second gradation data is equally divided into (N−1) ranges tocalculate a first intra-range gradation value that is a gradation valueof the first gradation data in a range including the first gradationdata and calculate a second intra-range gradation value that is agradation value of the second gradation data in a range including thesecond gradation data, and in a case where the range including the firstgradation data and the range including the second gradation data areequal, i) in a case where a sum of the first intra-range gradation valueand the second intra-range gradation value is equal to or smaller thanthe maximum value of the thresholds arrayed in the first thresholdmatrix, the first quantization data is generated based on a result ofcomparing the first intra-range gradation value with the first thresholdand the second quantization data is generated based on a result ofcomparing the second intra-range gradation value with the secondthreshold, and ii) in a case where the sum is greater than the maximumvalue, a first threshold offset amount and a second threshold offsetamount are generated by dividing a value that is obtained by subtractingthe maximum value from the sum into two, the first quantization data isgenerated based on a result of comparing the first intra-range gradationvalue with a value obtained by adding the first threshold offset amountto the first threshold, and the second quantization data is generatedbased on a result of comparing the second intra-range gradation valuewith a value obtained by adding the second threshold offset amount tothe second threshold.